One who is politically active in the role of a citizen; especially, one who campaigns for change. / One who is conspicuously active in carrying out any occupational or professional functions.

Behaving as an activist.

plural of activist

Relating to the practice of activism or the behavior and beliefs of activists.

A person who participates in direct action.

Of or relating to direct action.

plural of activist judge

(US, derogatory, chiefly in right-wing discourse) A judge or justice who makes rulings based on personal political views or considerations rather than on the law, or who issues rulings intended to have political effects.

plural of activist justice

(US, derogatory, chiefly in right-wing discourse) A justice (usually referring to a member of a Supreme, High or Appellate court) who makes rulings based on personal political views or considerations rather than on the law, or who issues rulings intended to have political effects.