For a general reaction, a A + b B --> c C + d D, the rate law expresses the relationship between the concentrations of the reactants raised to some power and the rate constant, a proportionality constant.

The powers n and m are NOT equal to the stoichiometric coefficients a and b in the reaction equation.

The powers n and m are called the order of A and B, respectively. For n = 0, the reaction is zeroeth order in A. For n = 1, the reaction is first order in A. For n = 2, the reaction is second order in A. The reaction order is the sum of n plus m. The overall reaction could be first or second order. (Third order reactions are possible but very rare.)

As an example, consider an expression in the following equation:

The reaction is first order in hydrogen (H₂) and ethylene (C₂H₄) but second order overall (i.e., the reaction order is 2 or second order).