In the US, impeachment means that Congress thinks the president is no longer fit to serve and should be removed from office. Here's what you need to know about what it is and how it works.

What is impeachment? The term "impeachment" itself dates back centuries in England, where it was "a device for prosecuting great lords and high officials who were beyond the reach of the law courts".

But in the US context, the framers of the ConstitutionA document that sets out the fundamental rules according to which a country is governed, which are usually difficult to change. set up the impeachment process as a way for CongressThe legislative branch of the American government, where elected representatives vote on which laws should be passed. Congress is made up of two chambers: the Senate, which contains two members from each of the 50 states, and the House of Representatives, which has representatives from 435 smaller districts. to remove the president from power.

First, the House of RepresentativesThe lower chamber of the United States congress. There are 435 representatives, with a certain number allocated to each state based on the state's population. has the power to impeach the president. A simple majority is necessary for an article of impeachment to be approved. (Each article lays out a charge against the president.)

Then the process moves to the SenateThe upper chamber of the Congress that - along with the US House of Representatives (lower chamber) - comprises the legislative branch of the US government. Each of the 50 US states is equally represented by two senators, regardless of its population size., where a trial will be held with the chief justice of the Supreme Court presiding.

Finally, and crucially, it takes a two-thirds vote from the Senate to actually convict a president on any count. Conviction on any count would then remove the president from office and put the vice-president in power.

Note that two-thirds of the Senate - 67 votes - is a very high threshold that is almost never achieved.

What can the president actually be impeached for? The Constitution specifies two specific crimes -
treasonThe crime of betraying your country. and bribery - that could merit impeachment and removal from office. In addition to that, it mentions a vaguer, broader category of "other high Crimes and Misdemeanors".

That's all we get, and just what that last category entails has been the subject of a great deal of debate throughout US history. When Gerald FordThe Republican Party US president from 1974 to 1977. was House minority leader, he said, "An impeachable offence is whatever a majority of the House of Representatives considers it to be at a given moment in history."

How often has impeachment happened in history? The only two American presidents ever to have been impeached were Andrew Johnson in 1868 and Bill ClintonThe American president from 1993 to 2001. He is a Democrat. in 1999. Neither was actually convicted and removed from office. However, Richard NixonThe 37th US president, who served from 1969 to 1974. He was a Republican. was headed towards seemingly certain impeachment and likely conviction in 1974, and preemptively resigned his office. No other president has come particularly close to being impeached.

How does impeachment actually play out in the House? Impeachment happens in the House of Representatives and, since the House is run on majority rules, it's really up the majority party to run the process as it sees fit.

How does an impeachment trial play out in the Senate? Though the actual action of impeachment in the House looks a lot like votes on any ordinary bill or resolution, the Senate is where things start to look quite different because the Senate is hosting a trial - something it very rarely does.

In this trial, the House of Representatives acts as a prosecutor, designating certain impeachment managers to argue their side in the Senate. The president's lawyers are the defence team - the president does not have to appear in person and, historically, has not done so. The chief justice of the Supreme Court presides and is responsible for making procedural rulings during the trial. However, the Senate can vote to overrule his or her decisions.

What does this mean for Trump? Crucially, Johnson, Nixon and Clinton - the presidents who were either impeached or who, in Nixon's case, resigned to avoid impeachment - all faced Congresses in which both chambers were controlled by their political enemies and who, therefore, wanted them out of power.

Trump, of course, does not. And so long as Republicans control the Senate, it's difficult to imagine any removal of Trump. The president is still quite popular among Republican voters, and party interests still need him to appoint conservative judges and sign conservative bills. So, Republicans in Congress have a strong incentive to give him the benefit of the doubt on any scandal or controversy in which he has some sort of plausible deniability - and even some where he may not.

Constitution - A document that sets out the fundamental rules according to which a country is governed, which are usually difficult to change.

Congress - The legislative branch of the American government, where elected representatives vote on which laws should be passed. Congress is made up of two chambers: the Senate, which contains two members from each of the 50 states, and the House of Representatives, which has representatives from 435 smaller districts.

House of Representatives - The lower chamber of the United States congress. There are 435 representatives, with a certain number allocated to each state based on the state's population.

Senate - The upper chamber of the Congress that - along with the US House of Representatives (lower chamber) - comprises the legislative branch of the US government. Each of the 50 US states is equally represented by two senators, regardless of its population size.

Treason - The crime of betraying your country.

Gerald Ford - The Republican Party US president from 1974 to 1977.

Bill Clinton - The American president from 1993 to 2001. He is a Democrat.

Richard Nixon - The 37th US president, who served from 1969 to 1974. He was a Republican.