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."