Half-Duplex: One way at a time data communication; both devices can transmit and receive data, but only one at a time.

Half-Life: The time in which half the atoms of a radioactive substance will have disintegrated, leaving half the original amount. Half of the residue will disintegrate in another equal period of time.

Handshake: An interface procedure that is based on status/data signals that assure orderly data transfer as opposed to asynchronous exchange.

Hardcopy: Output in a permanent form (usually a printout) rather than in temporary form, as on disk or display terminal.

Hardware: The electrical, mechanical and electromechanical equipment and parts associated with a computing system, as opposed to its firmware or software.

Harmonic: A frequency that is a multiple of the fundamental. See also Distortion and Non-Linearity.

Harmonic Distortion: A type of communications line interface that is caused by erroneous frequencies that are generated by non-linearities in the system.

HART: Highway Addressable Remote Transducer.

HDTV: High Definition Television.

Head Loss: The loss of pressure in a flow system measured using a length parameter (i.e., inches of water, inches of mercury).

Head Pressure: Pressure in terms of the height of fluid, P = yrg, where r = fluid density and y = the fluid column heights. Expression of a pressure in terms of the height of fluid, r = yrg, where r is fluid density and y = the fluid column height. g = the acceleration of gravity.

Header: The portion of a packet, preceding the actual data, containing source and destination addresses and error-checking fields.

Heat Sink: 1. Thermodynamic. A body which can absorb thermal energy. 2. Practical. A finned piece of metal used to dissipate the heat of solid state components mounted on it.

Heat Transfer: The process of thermal energy flowing from a body of high energy to a body of low energy. Means of transfer are: conduction; the two bodies contact. Convection; a form of conduction where the two bodies in contact are of different phases, i.e. solid and gas. Radiation: all bodies emit infrared radiation.

Heat Treating: A process for treating metals where heating to a specific temperature and cooling at a specific rate changes the properties of the metal.

Heat: Thermal energy. Heat is expressed in units of calories or BTU's.

Hertz (Hz): Units in which frequency is expressed. Synonymous with cycles per second.

Hexadecimal: Refers to a base sixteen number system using the characters 0 through 9 and A through F to represent the values. Machine language programs are often written in hexadecimal notation.

HEU: Highly Enriched Uranium.

Hit: For Internet sites, this is the common term for the numbers of times a site (or page) is accessed.

Hold: Meter HOLD is an external input which is used to stop the A/D process and freeze the display. BCD HOLD is an external input used to freeze the BCD output while allowing the A/D process to continue operation.

Hooke's Law: Defines the basis for the measurement of mechanical stresses via the strain measurement. The gradient of Hooke's line is defined by the ratio of which is equivalent to the Modulus of Elasticity E (Young's Modulus).

Host: The primary or controlling computer in a multiple part system.

Host Number: The part of an Internet address that designates which node on the (sub network is being addressed.

Hub: A central node in a star network to which all other nodes are connected by means of point-to-point communications links.

Humidity: The presence of water vapor in air or other gases Some people use "humidity" to mean relative humidity only. Strictly speaking, "humidity" also refers to all kinds of absolute indications of humidity. For very low humidities, other more specific terms tend to be used .

Hydrogen Ion Activity (aH+): Activity of the hydrogen ion in solution. Related to hydrogen ion concentration (CH+) by the activity coefficient for hydrogen (f H+).

Hygrometer: Any instrument for measuring humidity.

Hygrometry: The subject of humidity measurement.

Hygroscopic: Tending to absorb water vapor.

Hysteresis (Electrode Memory): When an electrode system is returned to a solution, equilibrium is usually not immediate. This phenomenon is often observed in electrodes that have been exposed to the other influences such as temperature, light, or polarization.

Hysteresis: The maximum difference between output readings for the same measured point, one point obtained while increasing from zero and the other while decreasing from full scale. The points are taken on the same continuous cycle. The deviation is expressed as a percent of full scale.