A terminator is the visible dividing line upon a planet's surface that separates day and night.
Commander Sulu stated that the HMS Bounty was crossing the terminator into night on approach to San Francisco, Earth during their visit to the 20th century. (Star Trek IV: The Voyage Home)
Lt. Worf gave the latitude and longitude of Paul Manheim's compound in relationship to Vandor IV's terminator. (TNG: "We'll Always Have Paris")