Illustrated Glossary of Organic Chemistry

Concerted reaction: A reaction in which all bond changes (new bonds formed and old bonds broken) occurs in a single mechanism step.

This Diels-Alder reaction is a concerted cycloaddition reaction. Two new sigma bonds and one pi bond are formed at the same time as three pi bonds are lost.

This SN1 reaction is not concerted because three mechanism steps (carbon-leaving group ionization, nucleophile capture, and deprotonation) are required to convert tert-butyl bromide (the reactant) into tert-butanol (the product).