That;s obviously one use of the word, but it's not the only one. It's activist when the Court says that Congress cannot do X, because it takes power away from the political branches and gives it to the unelected court.

There are examples of this that would not be activist - for example, if the Court were to strike down a law requiring citizens to house US soldiers in their homes - but generally the Constitution is imperfectly determinative, and so requires interpretation.

The non-activist position is not always the right or the wrong one, but the court is at its least activist where it gives effect to the expressed will of the political branches.