Something which is transient.

A transient phenomenon, especially an electric current; a very brief surge.

(acoustics) A relatively loud, non-repeating signal in an audio waveform which occurs very quickly, such as the attack of a snare drum.

Passing or disappearing with time; transitory.

“a transient pleasure”

Remaining for only a brief time.

“a transient disease”

Decaying with time, especially exponentially.