Illustrated Glossary of Organic Chemistry

Reactive intermediate: A reaction intermediate that has a short lifetime because it is unstable and reacts quickly. A reactive intermediate should not be confused with a transition state; a reactive intermediate is a saddle point on an energy profile whereas a transition state is an energy maximum.

The tert-butyl carbocation is a reactive intermediate. In this S_N1 reaction mechanism the tert-butyl carbocation is highly unstable. Once formed, it reacts quickly and cannot be isolated.

An energy profile showing the relationship between reactants, products, transition state, and reactive intermediate for the S_N1 reaction shown above.