\begin{definition}[Definition:Second-Order Reaction]
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'''.
\end{definition}