Rackmount

From DNBWiki

The Delta 1010 is an example of a rackmountable soundcard, although it is not racked in this picture. It is 1U in size.

Rackmount hardware is designed to fit into standard 19" racking. Almost everything can be found rackmountable - samplers, synths, modules, effects units, mixers, input channels, and so on.

Rackmount gear is generally measured in "units", so a small piece of gear is 1U (see example in picture) whereas large kit might require 3U, 4U (or even more) of rackmount space.