As you go down a group, the ionization energy decreases. This is due to the "shielding effect" which is basically when there is repulsion between the electrons in the inner shell and the valence electrons. As you go across a period, ionization energy increases. This occurs because as the number of protons increase in the nucleus, the attraction to the valence electrons also increase
The ionization energy (IE) of an atom or molecule describes the minimum amount of energy required to remove an electron (to infinity) from the atom or molecule in the gaseous state.[1]
X + energy → X+ + e-
The term ionization potential has been used in the past but is not recommended.