First, let’s start with what the Constitution defines as offenses warranting impeachment and removal from office. Those can be found in Article II, Section 4:
The President, Vice President and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors.
Treason and bribery are pretty straightforward, but what, exactly, constitutes high crimes and misdemeanors. Here’s what former House Minority Leader (and former president) Gerald Ford (R-Michigan) said about them:
An impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history; conviction results from whatever offense or offenses two-thirds of the other body considers to be sufficiently serious to require removal of the accused from office.