An activist judge is one who engages in judicial activism, forsaking the proper role of the judiciary in favor of advancing a particular (typically liberal) political agenda.
While some have decried the actions of conservative judges as "activism," these accusations are largely unwarranted. Typically, rulings by conservative judges simply correct liberal activism. In fact, by definition, a genuinely conservative judge will not engage in activism, as he will have a proper understanding of and respect for the role of the judiciary as envisioned by the Founding Fathers. By definition, then, any consistently activist judge is engaging in a liberal interpretation of this role.