Williamson synthesis of ether is an example of :

The Williamson ether synthesis is the most widely used method to synthesize ethers. It occurs by an ${S}_{N}2$ reaction in which a metal alkoxide displaces a halide ion from an alkyl halide.

${\text{RO}}^{-} + {\text{R}}^{'} - \text{x}\mathop{\to}_{{\text{S}}_{\text{N}}}\text{R} - \text{O} - {\text{R}}^{'}$

This is Nucleophilic substitution reaction.