A generally pejorative term used to describe the practice by which judges (typically high ranking judges, e.g. the United States Supreme Court) set groundbreaking precedents via excessively (from the perspective of those using the term) loose interpretations of the law. Such interpretations are most often thought to be politically motivated. Made possible by the legal doctrine of stare decisis, wherein judges are compelled by tradition to mirror rulings from similar past cases.
Roe v. Wade, in which the United States Supreme Court ruled that abortion could not be banned due to an implicit "right to privacy" in the Constitution, is the quintessential example of "judicial activism."
A term used by conservatives to describe court decisions which they disagree with. More often than not, applies to cases where the court declined the opportunity use its power, such as in the Terri Schiavo case, or Korematsu v. United States (1944).
In a Republican's mind, Lawrence v. Texas (2003) is an 'activist decision' while Bush v. Gore (2001) is not.
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)
When unelected and, therefore, unaccountable liberal judges use their power to force their agenda on the rest of us and overthrow the will of the voters, by making the law rather than enforcing the law. It is the tool that liberals use to erode the foundations of this country they don't like; liberals can't advance their agenda in the voting booth, so the try to to do it in the courts.
Judicial activism is becoming a real problem in America's judicial system today.