Multiple devices attached to one machine which operate independently of each other.

Multiseat X is where one machine is attached to several sets of keyboards, mice, and monitors, which can operate independently of each other -- each under the control of a different user. Contrast this with multi-head X, in which a single keyboard and mouse is associated with two or more screens. In theory, there are several different ways to achieve multi-seat; in practice there are very few approaches which are both simple and stable.