"judicial activism"

I recognize I'm a tiresome bore (not boor, I hope) on this topic, but would intelligent (and other) people, please, please stop talking about the evils of "judicial activism" simpliciter?  The commentary leading up to and now following Elena Kagan's nomination and confirmation has pivoted, as it has on many recent Supreme Court (and other) judicial nominations and confirmations, around the question of whether the nominee would be -- quod Deus avertat!! -- an "activist," the asserted *assumption* being that activism is both (a) defined and (b) very, very, very bad.   The first thing I don't see, though, is the argument behind (a).  What is activism?  What is the evidence concerning what our Constitution actually charges judges to do?  What *exactly* is prohibited? And why?  What makes some offical judicial act passive rather than "active"?  Moving on to assumption (b),  passivism is not *inherently* a virtue, not even of a judge -- as Aristotle made clear the day before the day before yesterday.  The opponents of "judicial activism" invoke the Constitution, but they rarely and barely explain, in the contexts I have in mind (as well as some others), what exactly they mean by such activism and exactly why it is prohibited (or just plain evil).  Respect for the Constitution, at least, requires precision with respect to the Article III power.  We all understand what is going on behind and through this talismanic treatment/invocation of "judicial activism," but we can do better.  And we would be better for doing so.