A buzzer beater is a successful shot made in basketball (or other timed sports) as the game clock expires, leaving zero seconds remaining in a period or game. It is often a dramatic, game-winning or game-tying shot that must leave the player’s hands before the buzzer sounds to count. 

The ball must be released before the clock hits 0:00. If it is still in the player’s hand when the buzzer sounds, it is a violation and does not count.

OED | WIKIPEDIA