The idea, central to the → Copenhagen Interpretation
of quantum theory, whereby at the moment of observation the
→ wave function changes irreversibly from a description of all of
the possibilities that could be observed to a description of only the event
that is observed. More specifically, quantum
entities such as electrons exist as waves until they are observed,
then "collapse" into point-like particles. According to the Copenhagen Interpretation,
observation causes the wave function to collapse. However it is not known what causes
the wave function to collapse. Same as → wave collapse.

In → quantum mechanics, the function of space and time that
satisfies → Schrodinger equation.
The square of the modulus of its amplitude at any point represents the probability of
finding a particle there.