alteration of a physical system from one state, or condition, to another. In atomic and particle physics, transitions are often described as being allowed or forbidden (see selection rule). Allowed transitions are those that have high probability of occurring, as in the case of short-lived radioactive decay of atomic nuclei. In three-millionths of a second, for instance, half of any sample of unstable polonium-212 becomes stable lead-208 by ejecting alpha particles (helium-4 nuclei) from individual atomic nuclei. Forbidden transitions, on the other hand, are those that have a high probability of not occurring. A strictly forbidden transition is one that cannot occur at all.

A transition may be forbidden by some basic conservation law such as the conservation of angular momentum, which inhibits light and other electromagnetic energy from being emitted in certain transitions within excited atoms and nuclei, or the conservation of electric charge, which strictly forbids electrons from decaying into even more elementary particles.