The word impeachment gets tossed around quite a bit when it comes to President Trump, as the special counsel investigation into Russian interference in the 2016 election continues.

So far, Trump himself has not been accused of collusion or any other crime, and even Democrats in Congress are not talking about starting impeachment proceedings, so talk of impeachment is pretty much theoretical.

But a good theoretical discussion starts with some historical context, and there's a new book out on past impeachments of not just presidents, but governors.

Jack O'Donnell wrote a chapter for 'The Politics of Impeachment' on one such case is New York.