Half Life: The length of time during which half a given number of atoms of a radioactive element will disintegrate.

Half Thickness: The thickness of absorbing material necessary to reduce the intensity of radiation by one-half.

HCI: Hydrochloric acid.

Heat of Fusion: The amount of heat needed to melt a unit mass or weight of a substance at its normal melting point.

Heat of Vaporization: Heat required to vaporize a unit mass or weigh of a liquid at its normal boiling point.

Hertz: The unit of frequency.

Hold Point: Point defined in an appropriate document, beyond which an activity must not proceed without the approval of a designated organization or authority.

Hunting: The tendency of a mechanical system to oscillate about a normal condition, or about the point of alignment.

Hydrometer: A measuring instrument for determining the specific gravity of a liquid or solid.

Hydraulics: Study of liquids in motion.

Hydrostatics: Study of liquids at rest.

Hygrometer: Measuring instrument for measuring the relative humidity of the atmosphere.

Hysteresis: The property of a measuring instrument where its response to a given stimulus depends of the sequence of preceding stimuli.