Definition:Second-Order Reaction

Let $\mathcal A$ and $\mathcal B$ be two chemical substances in solution which react together to form a compound $\mathcal C$.

Let the reaction occur by means of the molecules of $\mathcal A$ and $\mathcal B$ colliding and interacting as a result.

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

This in turn is jointly proportional to the quantities of $\mathcal A$ and $\mathcal B$ which have not yet transformed.

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

Also see

 * Law of Mass Action