Office of the President-Elect

The President-elect of the United States is the apparent winner, as ascertained by the General Services Administration Administrator, following the general election on Election Day in November, and the finalized winner when the votes cast in December by the Electoral College are counted by a joint session of Congress in early January.

During the period between the presidential election and the inauguration, the outgoing president is said to be a lame duck, while the incoming president heads a presidential transition to ensure a smooth handover of power.

An incumbent president who has won re-election is generally not referred to as a president-elect, as they are already in office and is not waiting to become president. Likewise, if a vice president succeeds to the presidency by way of the president's death, resignation or removal (via impeachment) from office, that person never holds the title of president-elect, as they would become president immediately.

Article II, Section 1, Clause 2 of the United States Constitution, along with the Twelfth and Twentieth Amendments govern the election of the U.S. President. The procedure is also regulated by federal and state laws. Under federal Law, the presidential electors (the members of the Electoral College) must be "appointed, in each state, on the Tuesday next after the first Monday in November, in every fourth year". Thus, all states appoint their electors on the same date, in November, once every four years. However, the manner of appointment of the electors is determined by the law of each State.

...
Wikipedia