Lattice enthalpy refers to the energy change when one mole of an ionic compound is formed from its gaseous ions, specifically the energy released when the ionic compound is formed from its constituents. It is not the energy associated with breaking bonds but rather the energy involved in the formation of the solid lattice structure. In the Born-Haber cycle for MgCl2, the correct notation is U, representing the lattice enthalpy for one mole of the compound. Understanding lattice enthalpy is crucial for calculating the stability and formation of ionic compounds. Clear definitions and correct usage in calculations are essential for accurate thermodynamic assessments.