Enthalpy of neutralization is defined as the enthalpy change when 1 mole of acid/base is completely neutralized by base/acid in dilute solution.
For strong acid and strong base neutralization net chemical change is
 $H^{+}\left(aq\right)+O{H}^{-}\left(aq\right)\to H_{2}O\left(l\right);{\Delta }_{r}H°=-55.84kJ/mol$
$\Delta H{°}_{ionization}$  of aqueous solution of strong acid and strong base is zero.
When a dilute solution of a weak acid or base is neutralized, the enthalpy of neutralization is some what less because of the absorption of heat in the ionization of the weak acid or base, for weak aid/base

$\Delta H{°}_{neutralization}=\Delta H{°}_{ionization}+\Delta H°\left(H^{+}+O{H}^{-}\to H_{2}O\right)$