In planetary science, tidal locking is a phenomenon whereby the
rotation period of a planetary body about its axis equals its
orbital period. This means that one hemisphere of the planet
always faces its parent or primary. An example of a tidally-
locked planet is Daled IV, which was described by Data, as a
planet that "does not rotate." He further explained that "one
side has constant night, the other constant daylight." He
surmised that it was because of tidal locking that the two
hemispheres on the planet developed disparate cultures, which
may have been the major cause of the civil war on that planet.
(TNG: "The Dauphin") Another example of a tidally-locked planet
is Dytallix B, in orbit around Mira Antliae. This uninhabited
planet was mined by the Dytallix Mining Company. Due to the
extreme temperatures on both the day and night sides of the
planet, the mines were located in the temperate zone between
these hemispheres. (TNG: "Conspiracy")