(music) Any two or more pitches or pitch classes all of which occur simultaneously, or at the same time. It may be thought of as a more general term for any possible chord. Any chord progression or harmonic progression is then a simultaneity succession, though not all simultaneity successions are harmonic progressions and not all simultaneities are chords.

In physics, the relativity of simultaneity is the concept that distant simultaneity – whether two spatially separated events occur at the same time – is not absolute, but depends on the observer's reference frame.