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 While many of the definitions are unchanged, some of the definitions may be out of date and some of the terms may no longer be in use in the Data Center industry.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

Device to transfer heat between two physically separated fluids
* Counterflow heat exchanger: Heat exchanger in which fluids flow in opposite directions approximately parallel to each other
* Cross-flow heat exchanger: Heat exchanger in which fluids flow perpendicular to each other
* Heat pipe heat exchanger: See heat pipe
* Parallel-flow heat exchanger: Heat exchanger in which fluids flow approximately parallel to each other and in the same direction
* Plate heat exchanger or plate liquid cooler: Thin plates formed so that liquid to be cooled flows through passages between the plates and the cooling fluid flows through alternate passages

Heat Exchanger, Rotary

See exchanger, rotary heat

Heat Load per Product Footprint

Calculated by using product measured power divided by the actual area covered by the base of the cabinet or equipment

Heat Load, Latent

Cooling load to remove latent heat, where latent heat is a change of enthalpy during a change of state

Heat Load, Sensible

The heat load that causes a change in temperature

Heat Pipe

Also defined as a type of heat exchanger. Tubular closed chamber containing a fluid in which heating one end of the pipe causes the liquid to vaporize and transfer to the other end where it condenses and dissipates its heat. The liquid that forms flows back toward the hot end by gravity or by means of a capillary wick

Heat Sink

Component designed to transfer heat from an electronic device to a fluid. Processors, chipsets, and other high heat flux devices typically require heat sinks

Heat, Total (Enthalpy)

A thermodynamic quantity equal to the sum of the internal energy of a system plus the product of the pressure-volume work done on the system: h = E + pv where h = enthalpy or total heat content, E = internal energy of the system, p = pressure, and v =volume. For the purposes here, h = sensible heat + latent heat. Sensible heat is heat that causes a change in temperature. Latent heat is the change of enthalpy during a change of state


High-efficiency particulate air

High-efficiency Particulate Air (HEPA) Filters

These filters are designed to remove 99.97% or more of all airborne pollutants 0.3 microns or larger from the air that passes through the filter. There are different levels of cleanliness, and some HEPA filters are designed for even higher removal efficiencies and/or removal of smaller particles

High-performance Computing and Communications (HPCC)

Includes scientific workstations, supercomputer systems, high speed networks, special purpose and experimental systems, the new generation of large-scale parallel systems, and application and systems software with all components well integrated and linked over a high speed network

High-performance Group (HPG)

One of several groups within the SPEC organization. HPG has created the benchmark suite SPEChpc96, aiming at high-end machines including both shared-memory and distributed-memory architectures

High-performance Steering Committee (HPSC)

Executive group within HPG. Currently, HPG and HPSC are the same (i.e., all HPG members are part of HPSC)

Horizontal Displacement (HDP)

An air distribution system used predominantly in telecommunications central offices in Europe and Asia. Typically, this system introduces air horizontally from one end of a cold aisle

Horizontal Overhead (HOH)

An air distribution system that is used by some long-distance carriers in North America. This system introduces the supply air horizontally above the cold aisles and is generally utilized in raised-floor environments where the raised floor is used for cabling

Hot Aisle/Cold Aisle (from ASHRAE)

A common means of providing cooling to data communications rooms in which IT equipment is arranged in rows and cold supply air is supplied to the cold aisle, pulled through the inlets of the IT equipment, and exhausted to a hot aisle to minimize recirculation of the hot exhaust air with the cold supply air

Hot Aisle/Cold Aisle (from ASHRAE, industry)

* A common arrangement for the perforated tiles and the data communications equipment. Supply air is introduced into a region called the cold aisle
* On each side of the cold aisle, equipment racks are placed with their intake sides facing the cold aisle. A hot aisle is the region between the backs of two rows of racks
* The cooling air delivered is drawn into the intake side of the racks. This air heats up inside the racks and is exhausted from the back of the racks into the hot aisle

Hot Gas

Pressurized gas leaving the compressor (discharge) prior to entering the condensing surface


High-performance computing


The process of adding moisture to air or gases


Water vapor within a given space
* Absolute humidity: The mass of water vapor in a specific volume of a mixture of water vapor and dry air.
* Relative Humidity: Ratio of the partial pressure or density of water vapor to the saturation pressure or density, respectively, at the same dry-bulb temperature and barometric pressure of the ambient air. Ratio of the mole fraction of water vapor to the mole fraction of water vapor saturated at the same temperature and barometric pressure. At 100% relative humidity, the dry-bulb, wet-bulb, and dew-point temperatures are equal

Humidity Ratio

The ratio of the mass of water to the total mass of a moist air sample. It is usually expressed as grams of water per kilogram of dry air (gw/kgda) or as pounds of water per pound of dry air (lbw/lbda)

Humidity, Relative

See relative humidity

HVAC System Efficiency

See efficiency, HVAC system

Hydrofluorocarbon (HFC)

A halocarbon that contains only fluorine, carbon, and hydrogen


A term pertaining to water used for heating or cooling systems

Hygroscopic Dust Failure (HDF)

Sulfate and nitrate salts to which water adheres and promotes corrosion

HyperText Transmission Protocol (HTTP)

The protocol over TCP/IP by which the worldwide web communicates. The specifications for HTTP are available from the World Wide Web Consortium, which develops such standards