Page "Energy level" Paragraph 38

Electrons can also be completely removed from a chemical species such as an atom, molecule, or ion.

Complete removal of an electron from an atom can be a form of ionization, which is effectively moving the electron out to an orbital with an infinite principal quantum number, in effect so far away so as to have practically no more effect on the remaining atom ( ion ).

For various types of atoms, there are 1st, 2nd, 3rd, etc.

ionization energies for removing 1, 2, 3, etc.

of the highest energy electrons from the atom in a ground state.