= Wave function collapse
<Wave function collapse> describes the conditional change of a <quantum state> associated with a measurement outcome. For an ideal <projective measurement> with projector $P_a$, the outcome probability is $p_a=\operatorname{Tr}(P_a\rho)$ and the conditional state is $P_a\rho P_a/p_a$ when $p_a>0$, according to the <Lüders rule>.
If the outcome is ignored, the state becomes $\sum_aP_a\rho P_a$, which is generally a mixture rather than any one conditional state. These are different information conditions. General measurement instruments can produce other state changes with the same outcome probabilities, so the ideal projection rule is a specified measurement model. Its physical interpretation is part of the <quantum measurement problem>.
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