Define the term oxidation, reduction, oxidation agent and reducing agent on the basis of oxidation number.

Oxidation: Oxidation is defined as a chemical reaction in which oxidation number of an element increases.
Reduction: Reduction is defined as a chemical reaction in which oxidation number of an element decreases.
Oxidising agent: Oxidising agent is a species which undergoes a decrease in oxidation number.
Reducing agent: Reducing agent is a species which undergoes an increase in oxidation number.
For example in the reaction:


The oxidation number of S increases from -2 to 0 while the oxidation number of Fe decreases from +3 (in FeCl₃) to +2 (in FeCl₂). Therefore H₂S is oxidised and FeCl₃ is reduced. Thus FeCl₃ acts as an oxidising agent and H₂S acts as a reducing agent.