A redox reaction is a type of chemical reaction in which there is a transfer of electrons between two or more species. The species that loses electrons is said to be oxidized, while the species that gains electrons is said to be reduced.

Example:

The following reaction is a redox reaction:

$$2Fe(s) + 3Cl_2(g) → 2FeCl_3(s)$$

In this reaction, iron (Fe) is oxidized from a neutral state to a +3 oxidation state, while chlorine (Cl) is reduced from a neutral state to a -1 oxidation state.