A metonymous term used to refer to synthesizers, samplers, mixers, effects boxes, and other music equipmenet that fits into a rackmount.

Rackmount height is measured in units. A unit is about 5cm or 2 inches high. Typical rackmount heights are 21U (about 1.1 meters) and 42U (about 2.1m). Different devices take differing amounts of units. Recently, 1U Pentium-III based machines have become available. Thjs means literally that you can fit 42 machines in one rack. That's great for Beowulf clusters and ISPs.