interpretation of quantum mechanics
An interpretation of quantum mechanics is a story about what the mathematics actually means — what is really going on in the world when an electron is in a superposition or a measurement gives a definite result. The equations themselves, above all the Schrodinger equation and the Born rule, are not in dispute: they predict the statistics of experiments with stunning accuracy. What is disputed is the picture behind those numbers.
The reason interpretations exist at all is the measurement problem. The wavefunction evolves smoothly and reversibly while no one is looking, yet a measurement seems to deliver a single, sharp outcome chosen at random. The bare formalism does not say where that switch from many possibilities to one fact happens, or whether it happens at all. Different interpretations answer that question in strikingly different ways.
It is worth being honest that, so far, the leading interpretations agree on every prediction we can actually test. Choosing between Copenhagen, many-worlds, pilot-wave theory, QBism and the rest is therefore partly a matter of which story you find clearest or least troubling, not which one experiment has crowned. They are not idle daydreaming, though: they shape how physicists think, suggest new questions, and occasionally point toward experiments that could one day tell them apart.
Interpretations agree on the testable numbers and differ on the picture behind them.
An interpretation is not a separate physical theory unless it makes a different prediction. Objective-collapse models do, in principle, which is why some people class them apart from mere interpretations.