A planetary class for worlds that cannot support humanoid life in the open but can be made habitable with pressure domes and life-support systems, such as the android world in I, Mudd.
Class K is the Federation designation for a planet whose natural surface is hostile to humanoids but which can be adapted for habitation by means of pressure domes or underground installations with their own life support.
The class is named on screen in The Original Series episode I, Mudd in 2268. When the android Norman hijacks the Enterprise and brings it to an uncharted world, Spock identifies the planet as Class K, noting that it is adaptable for humans with the use of pressure domes. The crew find that its inhabitants, some two hundred thousand androids built by a long-dead civilisation from the Andromeda galaxy, live in just such a sheltered complex under the self-appointed rule of Harry Mudd.
The designation illustrates how the classification system covers worlds between the comfortably Earth-like and the entirely uninhabitable.