What is Lattice enthalpy of ionic crystals?

It is defined as the energy released when one mole of ionic crystal is formed from its constituent ions in the gaseous state. The negative sign with lattice enthalpy shows that energy is released in the process. During the formation of ions the process is reversible and lattice enthalpy will have +ve sign because energy is absorbed in the process. Lattice enthalpy can also be defined as the energy required to completely separating one mole of solid ionic compound into gaseous ionic constituents.