Trump called it the ''i word." What is impeachment and how does it work?

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By Pete Williams  with NBC News Politics
Image: Protesters pass the White House during the Women's March in Washingt
Protesters pass the White House during the Women's March in Washington on Jan. 20, 2018.   -   Copyright  Mark Peterson Redux file

WASHINGTON — The congressional power to remove a president from office through the process known as impeachment is the ultimate check on the executive.

No president has ever been forced from the White House that way, though Richard Nixon resigned rather than have to face the prospect.

Congress gets the authority from the Constitution. The term "impeachment" is commonly used to mean removing someone from office, but it actually refers to the filing of formal charges.

The House impeaches. The Senate then holds a trial on those charges to decide whether the officer — a president or any other federal official — should be removed and barred from holding federal office in the future.

The House has impeached 19 people, mostly federal judges. Two presidents, Andrew Johnson and Bill Clinton, were impeached, but the Senate voted not to convict either of them.

Any member of the House can seek to start the process by proposing a resolution of impeachment. It would be referred to the House Judiciary Committee, which decides whether to investigate the allegations. If it does so, the committee would then vote on whether grounds for impeachment exist. A yes vote would report one or more charges, known as articles of impeachment, to the full House.

The Constitution provides that a president can be impeached for "treason, bribery, or other high crimes and misdemeanors." Treason and bribery are well understood, but the Constitution does not define "high crimes and misdemeanors."

Congress has identified three types of conduct that constitute grounds for impeachment, including misusing an office for financial gain. But the misdeeds need not be crimes. A president can be impeached for abusing the powers of the office or for acting in a manner considered incompatible with the office.

When Gerald Ford was a member of the House, he famously defined an impeachable offense as "whatever a majority of the House of Representatives considers it to be at a given moment in history." In other words, impeachment and conviction by Congress is a political punishment, not a criminal one.

Once the House Judiciary Committee acts, the full House votes. A simple majority of those present and voting is required to impeach. If the impeachment vote passes, House managers are appointed to serve as prosecutors in the Senate.

When a president has been impeached by the House, the Supreme Court's chief justice presides over the Senate trial. The office holder is entitled to defense lawyers, and each side can call witnesses and present evidence. When the case has been presented, the Senate meets in closed session to deliberate.

The Senate vote is conducted in open session, and a two-thirds majority is required for conviction. If the Senate votes to convict, the public official on trial is immediately removed from office. The Senate can conduct a separate vote on whether to disqualify the official from ever again holding federal office, which requires a simple majority.

The Senate's vote is final. The Supreme Court has said that because the Constitution gives the Senate "the sole power to try all impeachments," a conviction on impeachment cannot be appealed to any court.

In all of U.S. history, only eight people have been convicted after Senate impeachment trials, and all were federal judges.