Impeachment is a rarely used procedure that people often think means a president's removal from office. It actually means the House of Representatives has found evidence of impeachable conduct and is sending the process to the Senate for a trial that could lead to the president's removal.
Here's how it works:
It all starts in the House of Representatives. First, an official inquiry is opened in the House. Various committees may open investigations, but a panel made up of members known as the Judiciary Committee is ultimately responsible for recommending the articles of impeachment.
If the committee backs the impeachment articles, the matter goes to the full House for a vote.
If a majority of the full House votes for impeachment, the matter then goes to the Senate, which is responsible for holding a trial, overseen by the chief justice of the United States.
It takes a two-thirds vote in the Senate to force a president from office.
The Constitution dictates that if an impeached president is convicted by the Senate of "treason, bribery, or other high crimes and misdemeanors," he or she should be removed from office. There are no hard and fast rules on what constitutes a high crime or misdemeanor in the Constitution. It is left to Congress to define such terms.
If the senate did vote to oust the president from office, the vice president would take over.
No president has been removed from office as part of the impeachment process, though two have been impeached: Andrew Johnson in 1868 and Bill Clinton. Both won acquittal in the Senate.
Richard Nixon, who was the subject of impeachment proceedings, resigned from office in 1974 when it looked certain that the House would impeach him, and his prospects in the Senate appeared dire.