Illustrated Glossary of Organic Chemistry

Backside attack: In an ionic substitution reaction, approach of the nucleophile at a 180o dihedral angle to the existing (in SN2) or former (in SN1) carbon-leaving group bond. Causes inversion of configuration at the carbon suffering substitution.

Backside attack in the SN2 mechanism.

Backside attack on a carbocation in the SN1 mechanism.