Simultaneity

Simultaneity is the property of two events happening at the same time.

In mathematics, a system of equations or a set of simultaneous equations share variables; a solution is a set of variable values for which all these equations are satisfied simultaneously.
In music a simultaneity is more than one complete musical texture occuring at the same time, rather than in succession. This first appeared in the music of Charles Ives, and is common in the music of Conlon Nancarrow, and others. A pitch simultaneity is also more than one pitch or pitch class all of which occur at the same time, or simultaneously. Simultaneity 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.