Grounds for impeachment.
Here is what the Cornell Law School's on-line annotated Constitution says about the grounds for impeachment:
Impeachment, said Madison, was to be used to reach a bad officer sheltered by the President and to remove him “even against the will of the President; so that the declaration in the Constitution was intended as a supplementary security for the good behavior of the public officers.”752 The language of Sec. 4 does not leave any doubt that any officer in the executive branch is subject to the power; it does not appear that military officers are subject to it753 nor that members of Congress can be impeached.754
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The Convention came to its choice of words describing the grounds for impeachment after much deliberation, but the phrasing derived directly from the English practice. The framers early adopted, on June 2, a provision that the Executive should be removable by impeachment and conviction “of mal–practice or neglect of duty.”759 The Committee of Detail reported as grounds “Treason (or) Bribery or Corruption.”760 And the Committee of Eleven reduced the phrase to “Treason, or bribery.”761 On September 8, Mason objected to this limitation, observing that the term did not encompass all the conduct which should be grounds for removal; he therefore proposed to add “or maladministration” following “bribery.” Upon Madison’s objection that “
o vague a term will be equivalent to a tenure during pleasure of the Senate,” Mason suggested “other high crimes and misdemeanors,” which was adopted without further recorded debate.762 The phrase in the context of impeachments has an ancient English history, first turning up in the impeachment of the Earl of Suffolk in 1388.763
Treason is defined in the Constitution;764 bribery is not, but it had a clear common–law meaning and is now well covered by statute.765 High crimes and misdemeanors, however, is an undefined and indefinite phrase, which, in England, had comprehended conduct not constituting indictable offenses.766 In an unrelated action, the Convention had seemed to understand the term “high misdemeanor” to be quite limited in meaning,767 but debate prior to adoption of the phrase768 and comments thereafter in the ratifying conventions769 were to the effect that the President at least, and all the debate was in terms of the President, should be removable by impeachment for commissions or omissions in office which were not criminally cognizable. And in the First Congress’ “removal” debate, Madison maintained that the wanton removal from office of meritorious officers would be an act of maladministration which would render the President subject to impeachment.770 Other comments, especially in the ratifying conventions, tend toward a limitation of the term to criminal, perhaps gross criminal, behavior.771 While conclusions may be drawn from the conflicting statement, it must always be recognized that a respectable case may be made for either view.
http://www.law.cornell.edu/anncon/html/art2frag43_user.html
This short article is well researched and contains extensive footnotes as you can see from the numbers.