Terminator
From Memory Alpha, the free Star Trek reference.
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 gives latitude and longitude of Paul Manheim's compuond in relationship to Vandor IV's terminator. (TNG: "We'll Always Have Paris")
