An expression for the rate of reaction of a particular reaction in terms of concentrations of chemical species and constant parameters (normally rate coefficients and partial orders of reaction) only. For examples of rate laws see equations (1)-(3) under kinetic equivalence, and (1) under steady state.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1157 (https://doi.org/10.1351/pac199466051077)