Ionization is the name of the change: atoms or molecules becoming ions. A gas can go through ionization if a strong electric field, or certain kinds of light and radiation, knocks electrons off its atoms. Once those ions are there, the gas can carry a current. In the lab this is a careful, measured process. In ordinary life you are more likely to meet it on warning signs about “ionizing radiation” — the kind that can knock electrons off atoms in living cells, which is why it can do harm. The word is a noun. The doing-word is ionize. British spelling often writes ionisation, with an s; American spelling keeps the z.