Data center glossary

Facility types

Data center

The Congressional Research Service defines a data center as a physical facility that houses and runs large computer systems.[1]

Hyperscale data center

According to industry analysts cited by CRS, a hyperscale data center should have at least 5,000 servers, at least 10,000 square feet of space, and an electric power rating above 100 megawatts.[1]

Colocation data center

A colocation (or "managed") data center is owned and operated by a third-party company that rents space, equipment or services to other organizations, according to CRS.[1]

Enterprise data center

An enterprise data center is an on-premises data center that a company builds, owns and operates to house and manage its own IT infrastructure, according to CRS.[1]

Edge data center

Edge data centers are smaller data centers located physically closer to end users to reduce network delay and speed up content delivery, according to CRS.[1]

AI data center

CRS notes that Executive Order 14141 defined an AI data center as a data center used primarily to develop or operate artificial intelligence.[2]

GPU

CRS says a GPU is generally considered better than a CPU at handling AI computing tasks, and that current chip technology links hundreds of GPUs across multiple servers with high-speed connections.[2]

Power units and equipment

Megawatt (MW)

The U.S. Energy Information Administration defines a megawatt as one million watts of electricity.[3]

Gigawatt (GW)

EIA defines a gigawatt as one billion watts, or one thousand megawatts.[4]

Megawatt-hour (MWh)

EIA defines a megawatt-hour as one thousand kilowatt-hours or one million watt-hours.[3]

Nameplate capacity

EIA defines generator nameplate capacity as the maximum rated output of a generator or other power equipment under manufacturer-specified conditions, usually expressed in megawatts.[4]

Uninterruptible power supply (UPS)

EPA's ENERGY STAR program describes an uninterruptible power supply as battery backup that protects connected equipment such as computers and servers from power outages.[5]

Backup generator

EIA defines a backup generator as one used only for testing or in an emergency, such as a power shortage.[6] CRS says data centers typically have on-site backup generators designed to run during a grid outage.[7]

Substation

EIA defines a substation as facility equipment that switches, changes or regulates electric voltage.[8]

Transmission line

EIA defines a transmission line as the conductors, insulators, structures and equipment used to move large amounts of power at high voltage, usually over long distances to major substations or delivery points.[9] CRS describes the transmission system as high-voltage lines (above 100 kilovolts) that move power long distances, and the distribution system as low-voltage lines that deliver electricity to most homes and businesses.[7]

Grid and rates

Interconnection queue

CRS explains that power plant developers asking to connect to the grid are placed in an interconnection queue to wait for an interconnection study.[7]

Large load

FERC's large-load interconnection proceeding (Docket RM26-4) concerns how large loads, generally defined as electricity demand greater than 20 megawatts, connect to the transmission system.[10]

Large load tariff

NYU Law's State Energy & Environmental Impact Center defines a large load tariff as a set of rules for electric service between customers above a certain size and the utility serving them.[11] In April 2026 the Pennsylvania PUC said its model large load tariff provisions apply to customers over 50 MW individually or 100 MW in aggregate.[12]

Behind-the-meter generation

CRS describes behind-the-meter generation as power plants a data center developer buys or builds to self-supply electricity delivered directly to the data center without using the utility grid.[7]

Demand response

EIA defines demand response programs as incentive programs that encourage customers to temporarily reduce electricity demand at certain times in exchange for lower bills.[13] CRS says demand response programs typically pay customers to reduce electricity demand when the utility asks, usually when supplies are constrained.[7]

Flexibility (curtailable load)

CRS uses "flexibility" for a data center's ability to cut its demand during peak hours by pausing or slowing computing tasks or switching to backup power.[7]

Interruptible load

EIA defines interruptible load as customer load that, under contract, can be interrupted at the time of annual peak at the direct request of the system operator.[14]

Independent system operator (ISO)

EIA defines an independent system operator as an independent, federally regulated entity that coordinates regional transmission and ensures electric system safety and reliability.[14]

Efficiency and water metrics

IT equipment energy

The DOE Better Buildings 'Data Center Metering and Resource Guide' (hosted on LBNL's datacenters.lbl.gov) describes IT equipment energy as the energy used by all of a data center's IT equipment, such as compute, storage and network equipment.[15]

Total facility energy

The same DOE Better Buildings guide describes total facility energy as all IT equipment energy plus everything that supports the IT equipment, such as power delivery, cooling and lighting.[15]

Power usage effectiveness (PUE)

CRS describes PUE as the ratio of all the power a data center uses to the power used by its IT equipment alone.[1] NREL defines PUE as the ratio of the total power used by a data center facility to the power delivered to its computing equipment.[16]

Water usage effectiveness (WUE)

The Green Grid's WUE metric, measured in liters per kilowatt-hour, is meant to be used alongside PUE and carbon usage effectiveness to assess a data center's water, energy and carbon sustainability.[17]

Water withdrawal

USGS defines water withdrawal as water removed from the ground or diverted from a surface-water source for use.[18]

Consumptive use

USGS defines consumptive use as the part of withdrawn water that is evaporated, transpired, built into products, consumed, or otherwise not available for immediate reuse.[18]

Cooling

Evaporative cooling

A University of Georgia Extension publication says most large modern data centers use evaporative cooling because it is energy-efficient, with the downside that it directly consumes water.[19]

Open-loop cooling

Louisiana's Department of Conservation and Energy describes open-loop cooling as warm water from servers being cooled by direct contact with air, causing evaporation and continuous water consumption.[20]

Closed-loop cooling

Louisiana's Department of Conservation and Energy describes closed-loop cooling as water or coolant circulating in sealed pipes or coils that are never exposed to air.[20]

Reliability tiers

Tier I

Uptime Institute describes a Tier I data center as the basic capacity level, with infrastructure to support IT for an office setting and beyond.[21]

Tier II

Uptime Institute describes Tier II facilities as having redundant power and cooling components that allow better maintenance and protection against disruptions.[21]

Tier III

Uptime Institute describes a Tier III data center as concurrently maintainable, with redundant components and redundant distribution paths serving the critical environment.[21]

Tier IV

Uptime Institute describes a Tier IV data center as having several independent, physically isolated systems that serve as redundant components and distribution paths.[21]

Taxes and incentives

Payment in lieu of taxes (PILOT)

New York City's Department of Finance explains that under PILOT agreements, properties are exempt from property taxes and instead make a payment in lieu of taxes.[22] The New York State Comptroller describes a PILOT as a mechanism for collecting revenue to support municipal services, often included in agreements with developments receiving a tax benefit.[23]

Tax abatement

The Texas Comptroller defines a tax abatement (under Texas law) as a local agreement that exempts all or part of the increase in a property's value from taxation for up to 10 years.[24] Good Jobs First, a subsidy watchdog group, describes property tax abatements as subsidies that reduce or eliminate the taxes a company pays on its property.[25]

Sales and use tax exemption

Virginia exempts qualifying computer equipment bought by data centers from retail sales and use tax if the data center meets statutory investment and employment requirements, according to the Virginia Economic Development Partnership.[26]

Community benefits agreement

The U.S. Department of Energy defines a community benefits agreement as a legally binding, negotiated agreement between a developer and a community that sets out benefits the community receives in return for supporting or not opposing a project.[27]

Land use and approvals

Rezoning

Fairfax County, Va., describes a rezoning (zoning map amendment) as one of the tools for changing land use, which only its Board of Supervisors can approve.[28]

Proffer

Fairfax County defines a proffer as a written condition voluntarily offered by a property owner and accepted by the Board of Supervisors in a rezoning, which becomes legally binding.[29]

Special exception / special permit

Fairfax County describes special exception and special permit uses as land uses that can have undue impacts on or be incompatible with other uses and so need site-specific review.[29]

Special use

Wake County, N.C., describes a special use as a land use that might be appropriate in a zoning district if special standards ensure it is located, designed and operated in harmony with neighboring development.[30]

Conditional use permit

California's Governor's Office of Planning and Research describes a conditional use permit as a tool letting a city or county consider, through a public hearing, special uses not allowed as of right in a zoning district.[31]

Moratorium

The New York Department of State defines a land use moratorium as a local enactment that temporarily suspends the right to obtain development approvals while a community considers changes to its plan or land use rules.[32]

Annexation

The Municipal Research and Services Center (Washington state) describes annexation as a legislative action by which a city or town adds territory beyond its existing city limits.[33]

Sources

1. Data Centers and Their Energy Consumption: Frequently Asked Questions (R48646)
   Congressional Research Service (via EveryCRSReport.com) · 2026-05-12
   6 source excerpts

   “A physical facility that houses and runs large computer systems”

   “According to industry analysts, to be considered a hyperscale data center, a facility should contain at least 5,000 computer servers and a large scale of network equipment and occupy at least 10,000 square feet of physical space, with an electric power rating, sometimes referred to as power draw, exceeding 100 megawatts (MW; one megawatt is equal to 1 million watts).”

   “Other organizations, especially those lacking the space, staff, or IT resources, often choose to rent a space, equipment, or services within a colocation data center (also known as a "managed data center") owned and operated by a third-party company.”

   “For example, a large company may choose to build, own, and operate an on-premises data center (also known as an "enterprise data center") to house and manage its own IT infrastructure.”

   “Cloud computing service providers may also operate and maintain smaller data centers physically located closer to end users (called "edge data centers") to reduce network latency and data communication delay, speed up content distribution, optimize real-time, data-intensive workloads, and improve application performance and user experience.”

   “An expression or metric sometimes used to describe the energy performance of data centers is power usage effectiveness (PUE), which is the ratio of all the power used by the data center to the power used by just the IT equipment.”

2. Data Centers and Cloud Computing: Information Technology Infrastructure for Artificial Intelligence (IF12899)
   Congressional Research Service (via EveryCRSReport.com) · 2025-02-05
   2 source excerpts

   “In E.O. 14141, the term AI data center means a data center used primarily to develop or operate AI”

   “a GPU is generally considered better than a CPU to handle AI computational tasks ... Cutting-edge chip technologies also support high-speed (at the gigabit-per-second level) GPU-to-GPU data communication among hundreds of GPUs across multiple servers”

3. Glossary
   U.S. Energy Information Administration · 2026
   2 source excerpts

   “Megawatt (MW): One million watts of electricity.”

   “Megawatthour (MWh): One thousand kilowatt-hours or 1 million watt-hours.”

4. Glossary
   U.S. Energy Information Administration · 2026
   2 source excerpts

   “Gigawatt (GW): One billion watts or one thousand megawatts.”

   “The maximum rated output of a generator, prime mover, or other electric power production equipment under specific conditions designated by the manufacturer. Installed generator nameplate capacity is commonly expressed in megawatts (MW)”

5. Uninterruptible Power Supplies
   ENERGY STAR (U.S. EPA) · 2026
   Source excerpt

   “An Uninterruptible Power Supply (aka a UPS Battery Backup) protects vital connected equipment -- computers, servers, and telecommunications equipment -- from power outages.”

6. Glossary
   U.S. Energy Information Administration · 2026
   Source excerpt

   “Backup generator: A generator that is used only for test purposes, or in the event of an emergency, such as a shortage of power needed to meet customer load requirements.”

7. Data Centers and the Electricity Grid: Frequently Asked Questions (R49326)
   Congressional Research Service (via EveryCRSReport.com) · 2026-09-01
   6 source excerpts

   “Data centers also typically have backup generators on-site designed to operate in case of a grid outage.”

   “The transmission system consists of high-voltage (rated above 100 kilovolts) power lines and associated equipment that move power across long distances. The distribution system consists of low-voltage power lines and associated equipment that makes final delivery of electricity to most homes and businesses.”

   “Power plant developers requesting to interconnect to the grid are placed in an interconnection queue to wait their turn for an interconnection study to be completed.”

   “A third option for data center developers is to self-supply electricity by buying or building behind-the-meter power plants, also referred to as behind-the-meter generation. In this case, the electricity generated by the power plant is delivered directly to the data center without using the utility's electric grid.”

   “Demand response programs typically compensate electricity customers for reducing their electricity demand when their utility requests it, usually when utility electricity supplies are constrained.”

   “Some data centers have the technical potential to decrease their demand during peak demand hours, either by pausing or slowing computational tasks or by switching to backup power supplies. This behavior is known as flexibility”

8. Glossary
   U.S. Energy Information Administration · 2026
   Source excerpt

   “Substation: Facility equipment that switches, changes, or regulates electric voltage.”

9. Glossary
   U.S. Energy Information Administration · 2026
   Source excerpt

   “A set of conductors, insulators, supporting structures, and associated equipment used to move large quantities of power at high voltage, usually over long distances between a generating or receiving point and major substations or delivery points.”

10. Interconnection of Large Loads to the Interstate Transmission System (Docket No. RM26-4-000)
    Federal Energy Regulatory Commission · 2026
    Source excerpt

    “The ANOPR seeks public input on potential reforms related to how large loads — generally defined as electricity demand greater than 20 megawatts (MW) – interconnect to the transmission system.”

11. A New Regulatory Approach to Rising Electricity Demand: "Large Load Tariffs"
    State Energy & Environmental Impact Center, NYU School of Law · 2026-04-09
    Source excerpt

    “A large load tariff is a set of rules for electric service between customers over a certain size and the electric utility company that will serve them.”

12. PUC Acts to Protect Ratepayers, Guide Data Center Growth with New Large Load Tariff Framework
    Pennsylvania Public Utility Commission · 2026-04-30
    Source excerpt

    “Tariff provisions will apply to customers over 50 MW individually or 100 MW in the aggregate.”

13. Glossary
    U.S. Energy Information Administration · 2026
    Source excerpt

    “Incentive-based programs that encourage electric power customers to temporarily reduce their demand for power at certain times in exchange for a reduction in their electricity bills.”

14. Glossary
    U.S. Energy Information Administration · 2026
    2 source excerpts

    “The consumer load that, in accordance with contractual arrangements, can be interrupted at the time of annual peak load by the action of the consumer at the direct request of the system operator.”

    “An independent, federally regulated entity established to coordinate regional transmission in a non-discriminatory manner and ensure the safety and reliability of the electric system.”

15. Data Center Metering and Resource Guide
    U.S. Department of Energy Better Buildings (hosted by Lawrence Berkeley National Laboratory) · 2017-02
    2 source excerpts

    “IT equipment energy includes the energy associated with all of the IT equipment (e.g., compute, storage, and network equipment).”

    “Total facility energy includes all IT equipment energy as described above, plus everything that supports the IT equipment using energy, such as: Power delivery components...Cooling system components...Lighting.”

16. High-Performance Computing Data Center Power Usage Effectiveness
    National Renewable Energy Laboratory · 2026
    Source excerpt

    “PUE is the ratio of the total amount of power used by a computer data center facility to the power delivered to computing equipment.”

17. Water Usage Effectiveness (WUE): A Green Grid Data Center Sustainability Metric (White Paper #35)
    The Green Grid · 2011
    Source excerpt

    “The new water usage effectiveness (WUE™) metric—combined with the PUE and carbon usage effectiveness (CUE™) metrics—enables data center operators to quickly assess the water, energy, and carbon sustainability aspects of their data centers. ... The units of WUE are liters/kilowatt-hour (L/kWh).”

18. Water-Use Terminology
    U.S. Geological Survey · 2026
    2 source excerpts

    “Water removed from the ground or diverted from a surface-water source for use.”

    “The part of water withdrawn that is evaporated, transpired, incorporated into products or crops, consumed by humans or livestock, or otherwise not available for immediate use.”

19. Understanding How Data Centers Impact Surface and Ground Waters
    University of Georgia College of Agricultural & Environmental Sciences (UGA Extension) · 2026-06-10
    Source excerpt

    “A majority of large modern data centers use evaporative cooling because it is energy-efficient, especially in hot climates. The downside is that it directly consumes water.”

20. Open-Loop vs. Closed-Loop Cooling for Data Centers (fact sheet)
    Louisiana Department of Conservation and Energy · 2026
    2 source excerpts

    “Warm water from servers is cooled by direct contact with air, causing evaporation. ... Evaporation removes heat very efficiently. However, evaporation means continuous water consumption.”

    “Water or coolant circulates in sealed pipes or coils, never being exposed to air.”

21. Uptime Institute Tier Classification System
    Uptime Institute · 2026
    4 source excerpts

    “A Tier I data center is the basic capacity level with infrastructure to support information technology for an office setting and beyond.”

    “Tier II facilities cover redundant capacity components for power and cooling that provide better maintenance opportunities and safety against disruptions.”

    “A Tier III data center is concurrently maintainable with redundant components as a key differentiator, with redundant distribution paths to serve the critical environment.”

    “A Tier IV data center has several independent and physically isolated systems that act as redundant capacity components and distribution paths.”

22. Payment in Lieu of Taxes (PILOT) Agreements
    NYC Department of Finance · 2026
    Source excerpt

    “Through these agreements, properties are exempt from property taxes and instead make a Payment in Lieu of Taxes (PILOT).”

23. Property Tax Exemptions in New York State
    Office of the New York State Comptroller · 2013-10
    Source excerpt

    “A PILOT is another mechanism for collecting revenue to support municipal services. ... PILOTs are often included as part of an IDA agreement with a commercial or industrial development that is receiving a tax benefit for the duration of a project.”

24. What Is a Tax Abatement?
    Texas Comptroller of Public Accounts · 2026
    Source excerpt

    “A tax abatement is a local agreement between a taxpayer and a local taxing unit that exempts all or part of the increase in the value of property from taxation for a period not to exceed 10 years.”

25. Property Tax Abatements
    Good Jobs First · 2026
    Source excerpt

    “Property tax abatements, exemptions, and reductions are subsidies that lower the cost of owning real and business personal property by reducing or eliminating the taxes a company pays on it.”

26. Data Center Retail Sales & Use Tax Exemption
    Virginia Economic Development Partnership · 2026
    Source excerpt

    “Virginia offers an exemption from retail sales and use tax for qualifying computer equipment purchased by data centers that meet statutory investment and employment requirements.”

27. FAQ: Community Benefits Agreements
    U.S. Department of Energy · 2024-12
    Source excerpt

    “A community benefits agreement is a legally binding, negotiated agreement between a project proponent/developer and a community, often represented by a coalition of community group(s), outlining benefits they will receive in return for their support or non-opposition of a project.”

28. Rezoning Process
    Fairfax County (Va.) Department of Planning and Development · 2026
    Source excerpt

    “A zoning map amendment (also known as a rezoning) is one of the instruments by which a change in land use occurs. ... The rezoning of land can only be approved by the Board of Supervisors.”

29. Zoning Glossary
    Fairfax County (Va.) Department of Planning and Development · 2026
    2 source excerpts

    “A written condition, which, when offered voluntarily by a property owner and accepted by the Board of Supervisors in a rezoning action, becomes a legally binding condition.”

    “Uses, which by their nature, can have an undue impact upon or can be incompatible with other land uses and therefore need a site-specific review.”

30. Special Use
    Wake County (N.C.) Government · 2026
    Source excerpt

    “A land use designated as a 'special use' in a zoning district is one that might be appropriate if special standards are in place to ensure it is located, designed and operated in a manner that is in harmony with neighboring development.”

31. The Planner's Training Series: The Conditional Use Permit
    California Governor's Office of Planning and Research · 1997-07
    Source excerpt

    “A CONDITIONAL USE PERMIT (CUP) allows a city or county to consider special uses which may be essential or desirable to a particular community, but which are not allowed as a matter of right within a zoning district, through a public hearing process.”

32. Land Use Moratoria (James A. Coon Local Government Technical Series)
    New York Department of State · 2010
    Source excerpt

    “A land use moratorium is a local enactment which temporarily suspends a landowner's right to obtain development approvals while the community considers and potentially adopts changes to its comprehensive plan and/or its land use regulations to address new circumstances not addressed by its current laws.”

33. Annexation - Table of Contents
    Municipal Research and Services Center (MRSC) · 2024-02-23
    Source excerpt

    “Annexation by cities or towns is a legislative action whereby a city or town adds territory beyond its existing city limits.”

Definitions are quoted or closely paraphrased from the cited source. Where two authorities define a term slightly differently, both are shown.