What is "selection rule"

n. (context physics English) Any of several general rules concerning the transitions between the states of a quantum mechanical system; they derive from the symmetry properties of the states and of their interactions

In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in atomic nuclei, and so on. The selection rules may differ according to the technique used to observe the transition.

In the following, mainly atomic and molecular transitions are considered.