Legacy Glossary

This is a glossary of older terms. While many of the definitions are unchanged, some of the definitions may be out of date and some of the terms may no longer be in use in the Data Center industry. This legacy glossary provides definitions for hundreds of information and communications technology (ICT) and data center terms and acronyms. Arranged alphabetically and searchable, the glossary explains common industry vocabulary.
1 A B C D E F G H I K L M N O P R S T U V W Z
Absorbed Electrolyte

See electrolyte, absorbed


American Conference of Governmental Industrial Hygienists


Air changes per hour. Typically referring to outdoor air changes per hour


Generally, a measure of the noise level in an environment or from a sound source. For a point in an environment, the quantity is sound pressure level in decibels (dB). For a sound source, the quantity is sound power level in either decibels (dB) or bels (B). Either of these quantities may be stated in terms of individual frequency bands or as an overall A-weighted value. Sound output typically is quantified by sound pressure (dBA) or sound power (dB). Densely populated data and communications equipment centers may cause annoyance, affect performance, interfere with communications, or even run the risk of exceeding OSHA noise limits (and thus potentially cause hearing damage), and reference should be made to the appropriate OSHA regulations and guidelines (OSHA 1996). European occupational noise limits are more stringent than OSHA's and are mandated in EC Directive 2003/ 10/EC (European Council 2003)

Aggregation Link

An instance of a MAC-physical layer-medium physical layer-MAC entity between a pair of aggregation systems (see IEEE 802.3, Clause 43)

Aggregation Port

An instance of a MAC-physical layer entity within an aggregation system (see IEEE 802.3, Clause 43)

Aggregation System

A uniquely identifiable entity comprising, among other things, an arbitrary grouping of one or more ports for the purpose of aggregation. An instance of an aggregated link always occurs between exactly two aggregation systems. A physical device may comprise a single aggregation system or more than one aggregation system (see IEEE 802.3, Clause 43)

Agile Device

A device that supports automatic switching between multiple physical layer technologies (see IEEE 802.3, Clause 28)


Air-handling unit, which is a device used to condition and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system

Air Cooling

See cooling, air

Air Economizer

See economizer, air

Air Inlet Temperature

The temperature measured at the inlet at which air is drawn into a piece of equipment for the purpose of conditioning its components

Air Outlet Temperature

The temperature measured at the outlet at which air is discharged from a piece of equipment

Air Short-cycling

Air conditioners are most efficient when the warmest possible air is returned to them. When cooler-than-expected air is returned to the air conditioner, it will perhaps mistakenly read that as the space temperature being satisfied. This air short-cycling is because the air is not picking the heat from the space before returning to the air conditioner (see also Air, Bypass)

Air Space

Where the air space below a raised floor or above a suspended ceiling is used to recirculate information technology equipment room/information technology equipment area environmental air, the wiring shall conform to Article 645 of NFPA 70, National Electrical Code

Air, Bypass

Air diverted around a cooling coil in a controlled manner for the purpose of avoiding saturated discharge air. On an equipment room scale, bypass air can also refer to the supply air that short-cycles around the load and returns to the air handler without producing effective cooling at the load

Air, Cabinet

Air (typically for the purposes of cooling) that passes through a cabinet housing data communications equipment

Air, Conditioned

Air treated to control its temperature, relative humidity, purity, pressure, and movement

Air, Equipment

Airflow that passes through the IT or data communications equipment

Air, Return (RA)

Air extracted from a space and totally or partially returned to an air conditioner, furnace, or other heat source

Air, Supply

Air entering a space from an air-conditioning, heating, or ventilating apparatus

Air- and Liquid-cooled Electronics

See electronics, air- and liquid-cooled

Air- and Liquid-cooled Rack

See rack, air- and liquid-cooled

Air- and Liquid-cooled Server

See server, air- and liquid-cooled

Air-cooled Data Center (ASHRAE glossary)

See data center, air- and liquid-cooled

Air-cooled Electronics

See electronics, air-cooled

Air-cooled Rack

See rack, air-cooled

Air-cooled System

See system, air-cooled

Aisle, Cold

See hot aisle/cold aisle

Aisle, Hot

See hot aisle/cold aisle


The portion of a fire alarm control panel or a remote device attached to the fire alarm control panel that displays the information associated with a notification. Notifications may include alarm or trouble conditions


A discrepancy between the actual and desired characteristics of an item. This definition is derived from ANSI T1.416-1999 and ANSI T1.105-1995, which take precedence


American National Standards Institute


A collection of data items usually laid out linearly in memory for simple access with an integer index from the base of the array. Many scientific applications use arrays to contain the dataset that they are analyzing. The larger the dataset, the larger the array size needs to be to fit the data

Associate Member

A membership class available to non-profit organizations at a significantly reduced cost. Allows for access to the benchmarks under development on the condition of significant involvement in the development process. Each group has their own rules and requirements regarding associate membership. Contact SPEC for details

ASTM International

Formerly the American Society for Testing and Materials (ASTM)


Attachment unit interface


The algorithm that allows two devices at either end of a link segment to negotiate common data service functions (see IEEE 802.3, Clause 28)


A percentage value representing the degree to which a system or component is operational and accessible when required for use

Availability Date

The date upon which that part of the system becomes generally available, that is available to anyone willing to pay the appropriate price and take immediate delivery