In quantum physics, what term describes a measurement that changes a system from several possible states to one outcome?

In quantum physics, a measurement changing a system from several possible states to one outcome is called wave function collapse.

Before measurement, a quantum state can be represented as a superposition of alternatives. The wave function assigns amplitudes to those alternatives, and the Born rule converts their squared magnitudes into probabilities. In the collapse description, observing an outcome updates the state to the corresponding measurement state.

Collapse is part of the traditional measurement postulate, but its physical status is debated. In the Copenhagen family of views, it is associated with measurement and the transition from quantum possibilities to a recorded result. Many-worlds interpretations instead describe universal wave-function evolution without a fundamental collapse, while objective-collapse theories propose that collapse is a real physical process.

Decoherence is related but not identical. It explains why interference between alternatives becomes effectively inaccessible through interactions with the environment, yet by itself it does not select one unique result in every interpretation.