What term describes a quantum system existing in multiple states simultaneously?

The term for a quantum system existing in multiple possible states simultaneously is superposition. It means the system is described by a combination of allowed quantum states, with each component carrying an amplitude that determines measurement probabilities.

Superposition is a consequence of quantum mechanics’ linear mathematics. An electron, for example, can be in a state combining different positions, energies, or spin directions. When a measurement is made, the result is one definite outcome, with probabilities calculated from the state’s amplitudes. Before measurement, the system is not merely hiding a preselected classical answer.

This idea is often illustrated with Schrödinger’s cat, a thought experiment involving a cat linked to a quantum event. The example was designed to highlight the difficulty of applying microscopic quantum rules to everyday objects, not to suggest that real cats routinely remain visibly half-alive and half-dead.

Superposition is also the foundation of a qubit. Unlike a classical bit, which is 0 or 1, a qubit can combine both basis states until measurement. Interaction with the environment causes decoherence, which rapidly suppresses observable interference and helps explain why macroscopic objects appear classical.