When an acid reacts with a base, then a salt and water are formed. This is an exothermic reaction and heat is produced. This reaction is called the neutralization reaction.
Enthalpy of neutralization reaction is the change in the enthalpy, when 1 mole of water is formed during the chemical reaction. It is equal to the difference between the enthalpies of formation of the products and enthalpies formation of the reactants.
This can be measured in the lab by measuring the temperature difference between the reactants and product solution. The mass m of the solution is known and its specific heat is found. Then as we know the number of moles of water formed, we find enthalpy of neutralization, ΔQ/n = m s ΔT/n, n = number of moles of water formed