Rate Law
Definition
An equation showing how reaction speed depends on reactant concentrations: determined by experiment, never by just looking at the balanced equation.
Technical definition
A rate law expresses reaction rate as rate = k[A]^m[B]^n, where k is the rate constant and the exponents (reaction orders) are determined experimentally. Orders need not match stoichiometric coefficients except in elementary steps.
Example
For the decomposition of N2O5, experiments show rate = k[N2O5]: first order in the reactant.
Common confusion
You cannot write a rate law from the balanced equation. The orders must be measured; most reactions' orders differ from their coefficients.