In quantum mechanics, wave function collapse occurs when a wave function-initially in a superposition of several eigenstates-reduces to a single eigenstate due to interaction with the external world. This interaction is called an "observation".

Collapse is one of two processes by which quantum systems evolve in time.

Collapse is a black box for a thermodynamically irreversible interaction with a classical environment.

The other is the continuous evolution via the Schrödinger equation.