https://en.m.wikipedia.org/wiki/Acting_President_of_the_United_States

An acting president of the United States is an individual who legitimately exercises the powers and duties of the president of the United States even though that person does not hold the office in their own right. There is an established presidential line of succession in which officials of the United States federal government may be called upon to take on presidential responsibilities if the incumbent president becomes incapacitated, dies, resigns, is removed from office (by impeachment by the House of Representatives and subsequent conviction by the Senate) during their four-year term of office; or if a president-elect has not been chosen before Inauguration Day or has failed to qualify by that date.