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 While many of the definitions are unchanged, some of the definitions may be out of date and some of the terms may no longer be in use in the Data Center industry.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
Data Center

A building or portion of a building whose primary function is to house a computer room and its support areas. Data centers typically contain high-end servers and storage products with mission-critical functions

Data Center Availability

Probability that a data center will be operable at a future time (takes into account the effects of failure and repair/maintenance of the data center)

Data Center Energy Practitioner (DCEP)

The data center industry and DOE partnered to develop the Data Center Energy Practitioner (DCEP) Program. The DCEP training program certifies energy practitioners qualified to evaluate the energy status and efficiency opportunities in data centers. More information at https://datacenters.lbl.gov/dcep

Data Center Reliability

Probability that a data center system will be operable throughout its mission duration (only takes into account the effects of failure of the data center)

Data Center, Air-cooled

Facility cooled by forced air transmitted by raised floor, overhead ducting, or some other method; A data center with only air-cooled equipment

Data Center, Liquid- and Air-cooled

Data center with both chilled air and liquid available

Data Center, Liquid-cooled

Data center with only liquid-cooled equipment

Data Frame

Consists of the destination address, source address, length field, logical link control (LLC) data, PAD, and frame check sequence

Data Terminal Equipment (DTE)

Any source or destination of data connected to the local area network


Abbreviation for the data and communications industry


The set of inputs for a particular benchmark. There may be more than one dataset available for each benchmark each serving a different purpose (e.g., measurement versus testing) or configured for different problem sizes (small, medium, large, etc.

Daughter Card

Also called daughter board. A printed circuit board that plugs into another circuit board to provide extended feature(s). A daughter card accesses its parent card's circuitry directly through the interconnection between the boards. A mezzanine card is a kind of daughter card that is installed such that it lies in the same plane, but on a second level above its parent


Decibels referenced to 1.0 mW

Dead-end Service Rating (Valves)

Valves rated for dead-end service can be placed at the end of a pipe without a cap (i.e., with one end at atmospheric pressure) and will not have any leakage of fluid across the valve at the service pressure rating of the valve


The process of removing moisture from air

Dew point

The temperature at which water vapor has reached the saturation point (100% relative humidity)

Dew-point Temperature

See temperature, dew-point

Dichotomous Sampler

Piece of measurement equipment that collects airborne particulates and separates them by size for analysis

Dielectric Fluid

A fluid that is a poor conductor of electricity

Direct Expansion (DX) System

A system in which the cooling effect is obtained directly from the refrigerant. It typically incorporates a compressor, and in most cases, the refrigerant undergoes a change of state in the system

Disk Unit

Hard disk drive installed in a piece of data communications equipment, such as a personal computer, laptop, server, or storage product

Diversity (from ASHRAE)

A factor used to determine the load on a power or cooling system based on the actual operating output of the individual equipment rather than the full-load capacity of the equipment

Diversity (from ASHRAE, industry)

Two definitions for diversity exist, diverse routing and diversity from maximum:
* Systems that employ an alternate path for distribution are said to have diverse routing. In terms of an HVAC system, it might be used in reference to an alternate chilled water piping system. To be truly diverse (and of maximum benefit), both the normal and alternate paths must each be able to support the entire normal load.
* Diversity can also be defined as a ratio of maximum to actual for metrics such as power loads. For example, the nominal power loading for a rack may be based on the maximum configuration of components all operating at their maximum intensities. Diversity would take into account variations from the maximum in terms of rack occupancy, equipment configuration, operational intensity, etc., to provide a number that could be deemed to be more realistic


A group of computers and devices on a network that are administered as a unit with common rules and procedures. Within the internet, domains are defined by the IP address. All devices sharing a common part of the IP address are said to be in the same domain

Double Data Rate Memory (DDR Memory)

An advanced version of synchronous DRAM (SDRAM) memory now used in most servers. DDR-SDRAM, sometimes called SDRAM II, can transfer data twice as fast as regular SDRAM because it can send and receive signals twice per clock cycle

Double Precision

A level of floating point accuracy that usually requires twice the space for each value than does single precision, but provides considerably more precision. For most systems running the SPEC CPU tests from the OSG (e.g. CPU2000), double precision implies a 64-bit value


Refers to a type of air conditioning system that discharges air downward directly beneath a raised floor commonly found in computer rooms and modern office spaces


A period of time during which a system is not operational due to a malfunction or maintenance

Dry-bulb Temperature (DB)

See temperature, dry-bulb


A well in a piping system that allows a thermometer or other device to be inserted without direct contact with the liquid medium being measured

Dual In-line Memory Module (DIMM)

A small circuit board that usually holds memory chips. A single in-line memory module (SIMM) has card edge connections that are connected to the same signals on both sides of the PCB, whereas a DIMM has different signals on each side of the PCB

Dynamic Random Access Memory (DRAM)

The most commonly used type of memory in computers. A bank of DRAM memory usually forms the computer's main memory. It is called dynamic because it needs to be refreshed periodically to retain the data stored within