A reducing agent (also called a reductant or reducer) is the element or compound in a reduction-oxidation (redox) reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized". This means that there must be an "oxidizer"; because if any chemical is an electron donor (reducer), another must be an electron recipient (oxidizer). Thus reducers are "oxidized" and oxidizers are "reduced". For example, consider the following reaction:

2 [Fe(CN)₆]⁴⁻ + Cl₂ → 2 [Fe(CN)₆]³⁻ + 2 Cl⁻