What is the name of the effect where a quantum system's state changes upon measurement?

The effect where a quantum system’s state changes upon measurement is called wave function collapse.

Before measurement, a quantum state can be represented as a superposition of several possible outcomes. When an observable is measured, the standard textbook formulation says that the state abruptly reduces to one of the observable’s allowed eigenstates. The Born rule gives the probability of each result from the squared magnitude of its probability amplitude.

The idea became central to quantum theory’s measurement problem: the Schrödinger equation describes smooth, deterministic evolution, while measurements produce one definite result. Werner Heisenberg discussed state reduction in 1927, and John von Neumann later formalized measurement as a distinct process alongside ordinary Schrödinger evolution.

Wave function collapse is not accepted as a literal fundamental process by every interpretation. Many-worlds rejects collapse, while decoherence explains why alternatives cease to produce observable interference without by itself selecting one unique outcome. Thus, the term is the conventional answer for the textbook measurement postulate, not an interpretation-neutral description of all quantum theories.