The energy required to remove a valence electron from a gaseous atom or ion is called energy

Ionization potential

The energy required to remove an electron from a neutral isolated gaseous atom and convert it into a positively charged gaseous ion is called ionisation potential. It is also called ionisation energy. The electron to be removed is valence electron. It is most loosely bound electron. The ionisation potential corresponds to the process X (g) → X + (g) + e −