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 While many of the definitions are unchanged, some of the definitions may be out of date and some of the terms may no longer be in use in the Data Center industry.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

A device to stop or regulate the flow of fluid in a pipe or a duct by throttling

Vapor Barrier

A material or construction that adequately impedes the transmission of water vapor under specified conditions

Variable Frequency Drive (VFD)

A system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor

Variable Speed Drive (VSD)

A system for controlling the rotational speed of either an alternating current (AC) or direct current (DC) motor by varying the voltage to the electrical power supplied to the motor


Variable air volume


The property of a computer program, or program segment, that allows for the simultaneous execution of operations on different data values, thus making it possible to allocate the work to a set of operators and accomplish the work in parallel. One example of work that is very vectorizable is taking an entire matrix of values and multiplying each by 2. It is possible for different operators to work on different cells of the matrix at the same time. One example of work that is not vectorizable is adding to each item in an array the value of the preceding item in the array as each calculation is dependent upon the results of the preceding calculation so there is no way to perform the operations at the same time. Vectorization is only one sub-class - probably one of the most restrictive sub-classes - of parallelizable programming


* Vector Quantity: Denotes the simultaneous time rate of distance moved and the direction of a linear motion
* Face Velocity: Velocity obtained by dividing the volumetric flow rate by the component face area


The process of supplying or removing air by natural or mechanical means to or from any space. Such air may or may not have been conditioned

Vertical Overhead (VOH) Class

Refers to the delivery of air from overhead ductwork

Vertical Underfloor (VUF) Class

Refers to the delivery of air from an underfloor space, i.e., a raised-floor cavity


Common alternative to logical, often used to refer to the artificial objects (such as addressable virtual memory larger than physical memory) created by a computer system to help the system control access to shared resources

Virtual Machine (VM)

A self-contained operating environment that behaves as if it is a separate computer. For example, Java applets run in a Java virtual machine that has no access to the host operating system. This design has two advantages:
* System Independence: An application will run the same in any VM regardless of the hardware and software underlying the system
* Security: Because the VM has no contact with the operating system, there is little possibility of a program damaging other files or applications. The second advantage, however, has a downside. Because programs running in a VM are separate from the operating system, they cannot take advantage of special operating system features

Virtual Private Network (VPN)

The use of encryption in the lower protocol layers to provide a secure connection through an otherwise insecure network, typically the internet. VPNs are generally cheaper than real private networks using private lines but rely on having the same encryption system at both ends. The encryption may be performed by firewall software or possibly by routers

Virtual Server

* A configuration of a networked server that appears to clients as an independent server but is actually running on a computer that is shared by any number of other virtual servers. Each virtual server can be configured as an independent website with its own hostname, content, and security settings
* Virtual servers allow internet service providers to share one computer between multiple websites while allowing the owner of each website to use and administer the server as though they had complete control

VLA Battery

See battery, VLA


Virtual Bridged Local Area Network (see IEEE P802.1Q)

Volatile Organic Compounds (VOCs)

Organic (carbon-containing) compounds that evaporate readily at room temperature. These compounds are used as solvents, degreasers, paints, thinners, and fuels

VRLA Battery

See battery, VRLA