Substitution: Nucleophilic Second Order
Second order nucleophilic substitution is a concerted processes in which a nucleophilic Lewis base displaces a nucleofugal Lewis base from a Lewis acid centre.

In organic chemistry, SN2 processes usually occur at a delta+ sp3 carbon centre.

SN2 processes cause the tetrahedral carbon to invert like an umbrella via a five valent carbonium ion transition state.

For more information look in the Chemogenesis webbook sections on substitution reactions and substitution, transfer, abstraction, displacement (STAD).