reducing agent
element or compound that loses (or "donates") an electron to another chemical species in a redox chemical reaction; losing electrons,oxidized,"reduces" (are "oxidized" by) oxidizers (oxidizing agents)
en-GB/ɹɪˈdjuːsɪŋ æʒɒn/
en-US/ɹɪˈdusɪŋ ˈeɪd͡ʒənt/