a complicated involuntary reaction to a sudden unexpected stimulus (especially a loud noise); involves flexion of most skeletal muscles and a variety more..a complicated involuntary reaction to a sudden unexpected stimulus (especially a loud noise); involves flexion of most skeletal muscles and a variety of visceral reactions(noun.act)
(physics) an abrupt transition of an electron or atom or molecule from one quantum state to another with the emission or absorption of a quantum (noun.event)