n (chemistry) A reaction in which electrons are gained and valence is reduced; often by the removal of oxygen or the addition of hydrogen.
n (context|computability theory) a transformation of one problem into another problem, such as mapping reduction or polynomial reduction.
n (music) An arrangement for a far smaller number of parties, e.g. a keyboard solo based on a full opera.
n (philosophy|phenomenology) A philosophical procedure intended to reveal the objects of consciousness as pure phenomena. (See phenomenological reduction.)