Substitution: Nucleophilic First Order
First order nucleophilic substitution occurs when a nucleofugal Lewis base leaving group spontaneously leaves a carbon centre (often with solvent assistance) to form a planar carbenium ion which subsequently attacked and complexed by a nucleophilic Lewis base.

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