wavefunction collapse
Wavefunction collapse is the textbook way of describing what seems to happen the moment a quantum measurement gives a result. Before the measurement the system is in a superposition — a weighted blend of several possible outcomes. At the instant of measurement that blend is said to jump abruptly to just one of them, the one you actually observe, with all the other possibilities discarded. The smooth, spread-out wave suddenly becomes a sharp, single answer.
Crucially, which outcome you get is not determined in advance. The theory gives only the probabilities, through the Born rule: the chance of each result is the squared size of its amplitude in the original superposition. Collapse, in this telling, is random and instantaneous, and it cannot be reversed once it has happened. It is the step where the rich quantum 'maybe' becomes a plain classical 'is'.
It is important to be honest that collapse is a postulate, added by hand to make the theory match what we see, not something derived from the Schrödinger equation, which by itself only ever spreads possibilities out. Whether collapse is a real physical event, a mere bookkeeping update of our knowledge, or an illusion produced by entanglement with the environment is exactly the issue that different interpretations of quantum mechanics dispute.
A superposition with many parts jumps to a single outcome k, whose probability is the squared amplitude.
Collapse is not derived from quantum dynamics; the Schrödinger equation alone never produces it. Treat 'collapse' as a name for the puzzle, not an established physical mechanism — interpretations disagree on whether it is real.