Ionization enthalpy is the minimal quantity of energy which is demanded to get rid of the most loosely bound electron from a neutral isolated gaseous atom to form a cation. A cation is formed when an atom loses electrons. But for the removal electrons the atoms should be in isolated gaseous form. Though a gaseous atom is randomly separated but there exist some attractive forces between the atoms. To calculate the ionization enthalpy, it is practically impossible to isolate a single atom. By lowering the pressure, it is possible to minimize the force of attraction between the atoms. This is the reason why the term ‘isolated gaseous atom’ is used in the definition of the ionization enthalpy.

The most stable state of a gaseous atom is its ground state and in the ground state of the atom, it is very easy to remove the electron from the atom. Hence the term ground state is used in the definition.