The SN2 mechanism (substitution, nucleophilic, bimolecular) is a single elementary step in which the nucleophile attacks the carbon from the side opposite the leaving group while the leaving group departs. The three other substituents of the carbon flip over, like an umbrella turned inside out: the Walden inversion.
Chemistry · Glossary
What is SN2 mechanism, Walden inversion?
Also known as: SN2 mechanism · Walden inversion