What is Nucleophilic substitution, substrate?

A nucleophilic substitution is the replacement, on a saturated carbon, of a leaving group Y by a nucleophile Nu: $\mathrm{Nu^-} + \mathrm{R{-}Y} \to \mathrm{R{-}Nu} + \mathrm{Y^-}$. The compound R–Y is the substrate; the carbon bearing Y is electrophilic because the C–Y bond is polarised towards Y.