A substance or device used to dissipate or absorb unwanted heat, as from a manufacturing process (or, with lasers, from reflected rays).

Abbreviated HeNe. Laser in which the active medium is a mixture of helium and neon, which is in the visible range. Used widely in industry for alignment, recording, printing, and measuring, it is also valuable as a pointer or aligner of invisible CO₂ laser light.

Abbreviated Hz. The unit of measurement which expresses the frequency of a periodic oscillation in cycles per second.