Impeachment always means a process in which an official is charged with unlawful activity. In different countries this process may be followed by the removal of the official from office and by criminal or civil punishment. In the US the impeachment means being charged by the House of Representatives with the following trial in the Senate.

Until now, only 2 American presidents have been impeached. The first one to be impeached was Andrew Johnson, the 17th President of the USA. Bill Clinton was impeached 131 years later, but neither one of them was convicted by the US Senate.

Andrew Johnson was impeached on the 24th of February, 1868 for his "high crimes and misdemeanors" and unfair dismissal of Edwin Stanton, the US Secretary of War. The trial began on the 13th of March, but on the 26th of May the whole thing was ended with Johnson’s opponents' failure: they didn't achieve the two-thirds majority necessary to convict him.

More Info: www.senate.gov