The 22nd Amendment says: "No person shall be elected to the office of the President more than twice, and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the President more than once."

Technically, Trump has another (albeit far-fetched) option if he wants to get around the 22nd Amendment and serve a third term - he could go on a joint ticket as vice president, then take over from the successful running mate after they are elected, should they die, resign or be impeached and removed from office.