1. If, in a reaction, the oxidation number of an atom in a reactant decreases, the atom is said to undergo reduction.

eg. 1:

eg. 2:

eg. 3:

eg. 4:

2. If, in a species subjected to a reaction, the sum of oxidation numbers of atoms that participate in the overall reaction decreases, the species is said to undergo reduction.

eg. 1:

In 1, the sum of oxidation numbers of atoms that participate in the reaction decreases from 0 to -2; 1 is reduced.

eg. 2:

In 2, the sum of oxidation numbers of atoms that participate in the reaction decreases from -2 to -4; 2 is reduced.

eg. 3:

In 3, the sum of oxidation numbers of atoms that participate in the reaction decreases from -2 to -4; 3 is reduced.

eg. 4:

In 4, the sum of oxidation numbers of atoms that participate in the reaction decreases from -4 to -6; 4 is reduced.

In most reductions, the species reduced either gains hydrogen (eg. 1 and 3), loses oxygen (eg. 2), or both.  Traditionally, in casual usage, a reduction reaction involving neither is not referred to as a reduction.

see also oxidation