The Standard Enthalpy of Hydration, DH°hyd, 298, is the heat evolved when 1 mole of gaseous ions become hydrated (surrounded by water molecules), measured under standard conditions.
Al3+(g) + aq ® Al3+(aq) DH°hyd, 298 = -4613 kJ mol-1
The higher the charge on the ions and the smaller their size, the more exothermic the hydration enthalpy.
When the solvent is other than water the process is referred to as solvation, with the term enthalpy of solvation, DH°solv, 298.
See also Hess's Law | Enthalpy of Solution | Lattice Enthalpy.