Definition:
The minimum energy needed to remove an electron from the gas phase of an atom
Trends:
Ionization energy decreases down a group. This is because of the increasing orbital size, making the electrons further from the nucleus and thus easier to remove.
Ionization energy increases across a period. This is because of the increase in nuclear charge, making the electrons closer to a full shell and thus harder to remove.
Extra:
Always endothermic
Breaking the core electron increases ionization energy significantly