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 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.

See also:

- Equation
- Relativity of simultaneity
- Synchronization
- Musical set theory