Alkylation
Alkylation involves a nucleophilic Lewis base (normally a carbanion or an oxyanion) complexing with an alkyl carbenium ion or (more often) initiating an SN2 substitution at a delta+ alkyl carbon centre, R+ below.

Reagents which add delta+ alkyl functions are called alkylating agents. Reagents which add delta+ methyl functions are called methylating agents.