Simultaneity is the property of two events happening at the same time.
In music or music theory a simultaneity is more than one complete musical texture occuring at the same time. This first appeared in the music of Charles Ives, and is common in the music of Conlon Nancarrow, and others. A 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.