Illustrated Glossary of Organic Chemistry

Intermediate: In a chemical reaction or mechanism, any reacting species which is no longer starting material or reactant, and has not yet become product, and which is not a transition state. An intermediate differs from a transition state in that the intermediate has a discrete lifetime (be it a few nanoseconds or many days), whereas a transition state lasts for just one bond vibration cycle. An intermediate may be an unstable molecule (a reactive intermediate) or a highly stable molecule.

This SN1 solvolysis reaction has two intermediates (a carbocation; shown in red, and an oxonium ion, shown in purple) and three transition states (shown in blue).

In an energy profile, an intermediate appears at an energy saddle point whereas a transition state appears at an energy maximum.