reducing

A reducing agent (also called a reductant, reducer, or electron donor) is an element or compound that loses or "donates" an electron to an electron recipient (called the oxidizing agent, oxidant, or oxidizer) in a redox chemical reaction. The oxidation state, which describes the degree of loss of electrons, of the reducer increases while that of the oxidizer decreases; this is expressed by saying that reducers "undergo oxidation" and "are oxidized" while oxidizers "undergo reduction" and "are reduced". Thus, reducing agents "reduce" oxidizers by reducing (decreasing) their oxidation state while oxidizing agents "oxidize" reducers by increasing their oxidation state.
To clarify, in a redox reaction, the agent whose oxidation state increases, that "loses/donates electrons", that "is oxidized", and that "reduces" is called the reducer or reducing agent, while the agent whose oxidation state decreases, that "gains/receives electrons", that "is reduced", and that "oxidizes" is called the oxidizer or oxidizing agent.
A reducing agent is thus oxidized by an oxidizer when it loses electrons that are gained by this oxidizing agent, which itself is simultaneously reduced by the reducer.
In their pre-reaction states, reducers have extra electrons (that is, they are by themselves reduced) and oxidizers lack electrons (that is, they are by themselves oxidized). A reducing agent typically is in one of its lower possible oxidation states and is known as the electron donor. Examples of reducing agents include the earth metals, formic acid, oxalic acid, and sulfite compounds.
For example, consider the overall reaction for aerobic cellular respiration:

C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)The oxygen (O2) is being reduced, so it is the oxidizing agent. The glucose (C6H12O6) is being oxidized, so it is the reducing agent.
In organic chemistry, reduction usually refers to the addition of hydrogen to a molecule, though the aforementioned definition still applies. For example, the oxidizing agent benzene is reduced to cyclohexane in the presence of a platinum catalyst:

C6H6 + 3 H2 → C6H12Historically, reduction referred to the removal of oxygen from a compound, hence the name 'reduction'. An important example of this phenomenon occurred during the Great Oxidation Event, in which biologically−produced molecular oxygen (dioxygen (O2), an oxidizer and electron recipient) was added to the early Earth's atmosphere, which was originally a weakly reducing atmosphere containing reducing gases like methane (CH4) and carbon monoxide (CO) (along with other electron donors) and practically no oxygen because any that was produced would react with these or other reducers (particularly with iron dissolved in sea water), resulting in their removal.
By using water as a reducing agent, aquatic photosynthesizing cyanobacteria produced this molecular oxygen as a waste product.
This O2 initially oxidized the ocean's dissolved ferrous iron (Fe(II) − meaning iron in its +2 oxidation state) to form insoluble ferric iron oxides such as Iron(III) oxide − (Fe(II) lost an electron to the oxidizer and became Fe(III), meaning iron in its +3 oxidation state) that precipitated down to the ocean floor to form banded iron formations, thereby removing the oxygen (and the iron).
The rate of production of oxygen eventually exceeded the availability of reducing materials that removed oxygen, which ultimately led Earth to gain a strongly oxidizing atmosphere containing abundant oxygen (like the modern atmosphere). The modern sense of donating electrons is a generalization of this idea, acknowledging that other components can play a similar chemical role to oxygen.

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