Tidal locking occurred when a planet's orbit matched its year, meaning that one side always faced its star. This meant one side was always hot and bathed in sunlight and the other side was cold and always in darkness. This process tended to kill any life on the planet. New Rarga was a planet that was tidally locked. (PROSE: Ghost Devices) The First Doctor visited an Earth-like planet which was also tidally locked, although the exact term was not used. (COMIC: The Gyros Injustice)