Classically, one of a group of enzymes, now termed oxidoreductases (EC class 1), that bring about oxidation by the addition of oxygen to a metabolite or by the removal of hydrogen or of one or more electrons. Oxidase is now used for those cases in which O2 acts as an acceptor (of H or of electrons); those removing hydrogen are now termed dehydrogenases. For individual oxidases, see the specific names.