A half-reaction is a balanced chemical equation for either the oxidation or the reduction part of a redox chemical reaction, explicitly including exchanged Electrons. Match the electron counts before adding the two half-reactions; the net equation then contains no transferred Electrons. A half-reaction is bookkeeping, not necessarily an elementary reaction. See Florida State University, lab background: separate electrode reactions and Daniell cell.
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A half-reaction is a way of representing either the oxidation or reduction part of a redox (reduction-oxidation) reaction. In a redox reaction, one species is oxidized (loses electrons) and another is reduced (gains electrons). Half-reactions help to clearly illustrate these processes by separating the two components. Each half-reaction shows the transfer of electrons and can be balanced for mass and charge.