1. The amount of energy required to remove an electron from a specific atom or ion to an infinite point, generally expressed in electron volts and numerically equal to the ionization potential.
2. The energy required o remove completely the weakest bound electron from its ground state in an atom or molecule so that the resulting ion is also in its ground state.
3. Amount of energy required to remove an electron from an isolated atom or molecule.

There is an ionization potential for each successive electron removed, though that associated with removing the first (most loosely held) electron is most commonly used.

The ionization potential of an element is a measure of its ability to enter into chemical reactions requiring ion formation or donation of electrons and is related to the nature of the chemical bonding in the compounds formed by elements.