Gerald Ford was asked what an impeachable offense was in 1970? He replied that it was whatever a majority of the House of Representatives considers it to be at a given moment of history.
There is a wonderful article on the history of impeachment in this week's NewYorker, The Invention and Reinvention of Impeachment by Jill Lepore. Lets face it, the i word is on everybody's lips, might as well familiarize yourself with its history.
The concept actually dates to 1376 when the English Parliament attempted to wrest power from the King. And then apparently it became the rage; there were no less than ten impeachments in England between 1376 and 1450, when the King, to put a stop to it, simply stopped summoning the Parliament.
Lepore adroitly explains the historical significance of the House acting as accusers and the Senate as judges in impeachments. This article is non partisan and shines a lot of light on the matter. I would read it if you are interested. You might also take a look at this article by Robert Sommers, Hail Kleisthenes which explains how they took care of things in ancient Athens.
Every year at an assembly of 6000 citizens, the Athenian people would be queried if, in their opinion, there was a leader whose power was too great and a threat to democracy.That works too.
If the vote was affirmative that person would be exiled from Athens for a ten year period, without loss of property or civil rights, nor with any dishonor to his family. This power was called ostrakismos, which we now know as ostracism.