**One issue that arises in regards to measurement is the unresolved problem of how wave function collapse occurs, termed the measurement problem.**

Wave function collapse is defined as the process by which a wave function initially in superposition (in multiple states at once) appears to reduce to a single state after interaction with the environment.

In other words, it is the condensation of physical possibilities into a single occurrence. While a physical system can be described by its wave function, a systems wave function cannot be directly observed without initiating wave function collapse. This measurement problem is a central issue in the interpretation quantum mechanics.