An atmospheric effect, where the particles produced evenly distribute throughout the venue, with no discernible clouds or concentrations. Most often used (extensively in Rock 'n' Roll) to produce better visibilities of the light beams.
The "old school" method of producing haze was to use a heater coil to burn a powder, sal ammoniac, to release particles in the air. Today, mineral oil or glycerin hazers such as the ReelEFX DF-50, Le Maitre Radiance, and many others, are used.
In this standards document, ANSI E1.23 - 2006 Entertainment Technology - Design and Execution of Theatrical Fog Effects, ESTA defines the terms thusly:
See also smoke, fog.