A judge who fails to implement the legislator's meaning of a language term in a U.S. federal law.
How to spot a judicial activist? In a court issued opinion, look at the source of the evidence that the judge uses to give meaning to a language term in a federal statute. If the source of the evidence is (i) dictionary definitions, (ii) a previous court opinion written by a judicial activist, (iii) the writings of legal scholars, or (iv) the judges personal beliefs, then you have just found an example of a judicial activist. However, if the source of the evidence is the U.S. Congressional Record and the evidence is properly considered based on timing, weight, and order of analysis, then that judge will have implemented the legislator's meaning of a language term in a law per Article I, section 8, clause 18 of the U.S. Constitution (The 'Congress' Power To Make All Laws' clause) and is not considered a judicial activist.
Democratic senator Russ Feingold of Wisconsin was quoted as saying, "At this point, perhaps we should all accept that the best definition of a judicial activist is a judge who decides a case in a way you don’t like."
Feingold's remarks come in response to the wave of Republican accusations that Judge Sonya Sotomayor, who is appearing before Supreme Court Confirmation hearings as I write this, is impartial and allows subjectivity to be brought into the court room, due to her affiliations with political groups and comments she has made in the past.
Johnny: gosh, that woman is such a judicial activist Jamie!
Jamie: yeah, but federal judges from both sides of the political spectrum can attest to her impartiality and fair rulings.
Johnny: yeah, so!