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")