The failure of a judge to implement the legislator's meaning of a language term in a law.
How to spot judicial activism? 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 from dictionary definitions, previous court opinions, the writings of legal scholars, or the judges personal beliefs, then you have just found an example of judicial activism. However, if the source of the evidence is from the 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)