Stinger
Also said as Musical sting, Hit point, Musical punctuation
A short burst of music played over the score to mark a moment: a discovery, an alarm, a victory. It is written to fit the score, and can wait for the next beat.
It lets the score react to an event instantly without changing what it is playing. On the beat it sounds composed. Off the beat it responds faster, but it can clash.
Say it
“When the player is spotted, play a brass stinger in A minor on the next beat, at most 300 ms late. The loop carries on underneath it.”
Where you’ve seen it
Metal Gear Solid, in 1998, pairs a "!" over a guard's head with a short, sharp alert sound when you are spotted, and the sound has barely changed through the series since.
Not to be confused with earcon
An earcon is an interface signal and sounds the same anywhere. A stinger is a piece of the music, and matches the score's key and tempo.