What is an activity series in chemistry?

In chemistry, an activity series is a list of substances ranked by their relative activity. This series is helpful in determining oxidation numbers in oxidation-reduction reactions as well as predicting the products of metal displacement reactions.

An activity series is often used in determining the direction of a redox reaction, which is a reaction in which the oxidation states of atoms are changed in the reactants and products. Oxidation is a loss of electrons, which represents an increase in the oxidation state of a molecule, atom or ion; reduction is a gain in electrons, which represents a decrease of the oxidation state.