Redox reactions involve changes in oxidation number.

Key terms include oxidizing agent (oxidant), the entity that brings about oxidation and becomes reduced, and reducing agent (reductant), the thing that brings about reduction and becomes oxidized.

The terms are counterintuitive. The reducing agent gives off electrons. Reduction, a process that implies loss, involves a gain of electrons. What actually is "lowered" is the oxidation number of one or more atoms.