Definition:Second-Order Reaction

Definition
Let $\AA$ and $\BB$ be two chemical substances in solution which react together to form a compound $\CC$.

Let the reaction occur by means of the molecules of $\AA$ and $\BB$ colliding and interacting as a result.

Then the rate of formation of $\CC$ is proportional to the number of collisions in unit time.

This in turn is jointly proportional to the quantities of $\AA$ and $\BB$ which have not yet transformed.

Such a chemical reaction is called a second-order reaction.

Also see

 * Law of Mass Action