Clinton was found guilty of perjury and obstruction of justice by the House of Representatives but then acquitted by the Senate. The impeachment needs to go through both houses. Nixon would have been inevitably impeached but he resigned rather than suffer the indignity of being the first ever successfully impeached president. There hasn't been a president that has been "successfully" impeached, but Trump strikes me as the kind of guy that would ride an impeachment all the way to the end.
Andrew Johnson and Bill Clinton are the only two presidents to have been successfully impeached by the House of Representatives, and both were later acquitted by the Senate.
https://en.wikipedia.org/wiki/Impeachment_in_the_United_States
Technically Clinton was successfully impeached by the US Congress. Approval was needed by the Senate but it does not mean that Clinton was not impeached.