What is judicial activism?

According to today's news stories, the President cast a potential Supreme Court decision against the constitutionality of his signature health care law as an action that would be tantamount to "judicial activism."  Both The New York Times ("President Confident Health Law Will Stand," page A17) and The Wall Street Journal ("Obama Warns Supreme Court," page A1) report on the matter.

I do not know how the nine justices will rule on the pertinent matters before the court.  And I am as opposed as anyone in principle to "judicial activism."  But is it really proper to frame judicial activism as a court's deciding whether legislation passed by the Congress adheres to the limits on federal power relative to state power and individual liberty as set forth in the governing documents of a republic?  Is that what judicial activism is?  Isn't that just judicial review according to Marbury v. Madison (1803)?  Aren't courts supposed to do that?

Or is judicial activism merely the term that we use to disparage a court's (potentially) ruling on the constitutionality of a matter in a way that we don't like?