A term describing the reaction rate of a chemical reaction in which the rate is proportional to the product of the concentrations (in moles) of two of the reactants (also called "bimolecular kinetics"), or to the square of the molar concentration of the reactant if there is only one. Such a reaction might have an equation like "rate = k[A][B]" or "rate = k[A]2," where k is the reaction rate constant, [A] is the concentration of reactant A, and [B] is the concentration of reactant B.