It is the enthalpy of a neutralization reaction (between an acid and a base forming salt and water). The reaction is exothermic.
It is the variation of enthalpy verified in the neutralization of 1mol H + of acid by 1mol OH- of the base, being all substances in total or infinite dilution, at 25 ° C and 1atm.
It is the variation of enthalpy involved in dissolving 1mol of a given substance in an amount of water sufficient to dilute the obtained solution.
When a solid is poured into a glass of water, it dissolves. In this order, it happens: