The point at which something begins.
Originally, the point on the land or water surface below which a nuclear bomb detonates in the air; now also the point on such a surface at or above the detonation.
The location of any disaster or violent assault.
The opening sequence of any event.
The opening kick of each half of a game of football.
The opening sequence of notes in a song, usually played by the song's main instrument, to indicate the beginning of the song.