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An atom's ionization potential or ionization energy is the energy (binding energy) required to remove an electron, thus ionizing it. For example, hydrogen (in the ground state) has an ionization potential that is about 13.6 eV (which is the Rydberg unit).
The term work function has a similar meaning, but is specific to removing an electron from the surface of something metal ("everyday" metal, such as iron), taking into account the effects of the crystal structure, the surrounding atoms, etc.