dɪˈkɹiːs dɪmɪˈnjuːʃ(ə)n ɹɪˈdʌkʃən ˌsɪmpləfəˈkeɪʃən the act of reducing complexity the act of decreasing or reducing something any process in which electrons are added to an atom or ion (as by removing oxygen or adding hydrogen); always occurs accompanied by oxidation of the reducing agent