Beyond its use as a standard label for any high court decision that doesn’t go their way, liberals use the phrase “judicial activism” to describe high court rulings that strike down some law of Congress as unconstitutional. As Walter Lippmann said of the “gradual collectivists” of his day, they reject any limits on the power of their transient majorities. So when a high court strikes down one of their pet laws, it’s as if democracy itself has been struck down. Their use of the phrase “judicial activism” is apt because they would prefer judicial “inactivism”; in other words, they would prefer if the Court just stopped performing the function of judicial constitutional review altogether, because who cares what those old Framers thought two centuries ago, etc. In liberal usage “judicial activism” is when judges deny legislators or the executive the power to do something unconstitutional. But that is what courts are supposed to do, so in that sense the phrase just means that the judges in question showed up for work.

The term is defined in different ways by different “authorities,” but Black’s Law Dictionary defines the term the way I’ve always used it: Judicial activism is when ”judges allow their personal views about public policy, among other factors, to guide their decisions.” It’s hard to do that when you’re strictly interpreting the text of a statute or constitutional provision. But it’s easy to do that when you’re legislating, because allowing personal views on public policy to affect decisions is precisely what legislators do. 

In the interest of maintaining diction discipline among conservatives, let’s agree that “judicial activism” means “judicial legislation” – the phrase used by Justice William Rehnquist in his Roe v. Wade dissent. Judicial activism is when judges cross the line that separates judging from legislating — as when the Supreme Court invents “constitutional rights” out of thin air, despite the fact that the right in question is not even hinted at in the text of the Constitution, and was totally unknown at the time that the Constitution (or, in the case of Roe, the 14th Amendment) was enacted.

Judicial activism, properly so-called, is when a high court takes some legislative power away from legislators. For example, ruling that gay marriage is a constitutional right would be “judicial activism.”