Order of Reaction
Definition
The exponents in the rate law, showing how strongly each reactant's concentration affects speed. They must be measured, never guessed from the equation.
Technical definition
Reaction order is the exponent of a reactant's concentration in the experimentally determined rate law. The overall order is the sum of individual orders, and orders may be zero, integer, fractional, or even negative.
Example
If doubling [A] quadruples the rate while [B] is held constant, the reaction is second order in A.
Common confusion
Order and molecularity are different. Order is an experimental number from the rate law, while molecularity counts particles colliding in a single elementary step.