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:

The reducing agent in this reaction is ferrocyanide ([Fe(CN)₆]^4-). It donates an electron, becoming oxidized to ferricyanide ([Fe(CN)₆]^3-), simultaneously the oxidizer chlorine is reduced to chloride.

In organic chemistry, reduction more specifically refers to the addition of hydrogen to a molecule, though the aforementioned definition still applies. For example, benzene is reduced to cyclohexane in the presence of a platinum catalyst:

In organic chemistry, good reducing agents are reagents that deliver H₂.