A President-elect is a political candidate who has been elected president but who has not yet taken office, as it is still occupied by the outgoing president. Similar terms can be used depending on the type and level of government, including Prime Ministers-elect, mayors-elect, governors-elect, etc.
In the United States, the members of the U.S. Electoral College are elected by the people in November once every four years; in December, they are in session and in turn elect the President of the United States; finally, the President of the United States assumes office in January. One is officially the president-elect only after being chosen by the Electoral College, but unofficially the person chosen in the November popular election is called the President-elect even before the Electoral College meets. An example of the practical effect of the official status is found in the U.S. Constitution's provision that if the President-elect dies, then the Vice President-elect becomes president on Inauguration Day. That rule takes effect only after the meeting of the Electoral College. If the person unofficially called the President-elect dies before that meeting, then the Electoral College would have broad discretion to choose some other person.
Nevertheless, there is a nearly ubiquitous perception that a president is chosen by the general elections and not by the electoral college, which did much to power the consternation and impatience for quickly resolving the situation involving the Florida recount during the Bush/Gore election in 2000.
US Presidential elections are held in November, but the President's term of office does not expire until January 20 of the following year. Presidents, and/or other politicians, will usually assemble a "transition team" of some sort, to prepare for a smooth transfer of power following the inauguration. The President often works closely with the President-elect on important policy matters during the last three months of the President's term, so as to ensure a smooth transition and continuity of operations that have significant national interests. The historical failure of James Buchanan to do this when he was President and Abraham Lincoln was the President-elect is considered one of the reasons leading up to the American Civil War. The problem was made worse by the fact that before 1936/37, the President-elect did not assume office until March, five months after the popular election.
During this time, the President is referred to as a lame duck because he has already accomplished about as much as he possibly can to promote the policies and agendas of his administration, and is usually not in a position to make very substantive decisions.
At noon, January 20 following a presidential election year, the term of office of the President expires by Constitutional mandate, and the President-elect becomes the President of the United States. This procedure has been the subject of many misinterpretations and urban legends, such as the David Rice Atchison presidency theory, which is not only predicated upon false assumptions but is also logically flawed. The formal oath of office does not affect the automatic accession to and occupation of the office of the presidency, which, in the case of the U.S. President, proceeds, ipso facto, from the expiration of the predecessor's term.