A reaction rate is the speed at which reactants are converted into products in a chemical reaction. The reaction rate is given as the instantaneous rate of change for any reactant or product, and is usually written as a derivative (e. g. d[A]/dt) with units of concentration per unit time (e. g. mol L-1 s-1).

(Sometimes referred to simply as rate.) The change with time in the concentration of a reactant or product involved in a chemical reaction. The rate of the reaction is given by the product of the rate coefficient for the particular reaction and the concentrations of the reactants.

The reaction rate or rate of reaction for a reactant or product in a particular reaction is intuitively defined as how fast a reaction takes place. For example the oxidation of iron under the atmosphere is a slow reaction which can take years, the combustion of butane in a fire is a reaction that takes place in fractions of a second.