David Davenport, a fellow at the Hoover Institution, Stanford University's policy think tank, for Forbes:
"...What is judicial activism? There are lots of opinions on that, but none is definitive. Apparently the origin of the term came not in a legal opinion at all, but in a 1947 Fortunemagazine article by historian Arthur Schlesinger in which he described the sitting U.S. Supreme Court as having 4 judicial activists, 3 judges who practiced “self-restraint,” and 2 in the middle. The distinction, Schlesinger argued, was based on their legal worldview, with the activists finding the law more malleable and subject to interpretation, whereas those engaged in self-restraint felt that legal terms had real meaning and it was not their place to provide a lot of interpretation.
... Since then, there have existed many understandings of judicial activism. Perhaps the most basic is when a court usurps the role of one of the other branches of government and takes up the work of the legislature or executive. Indeed, Justice William Rehnquist inRoe v. Wade found the majority of the Court engaged in judicial activism or “judicial legislation.” Another variation is when a judge is results-oriented, wanting to reach a particular conclusion and searching far and wide to find some strained legal interpretation to support it. Black’s LawDictionary says it is a “philosophy of judicial decision-making whereby judges allow their personal views about public policy, among other factors to guide their decisions.” I would submit that when a Court becomes an engine of change, rather than a brake on the illegal actions of another branch, it is engaging in judicial activism.
Under virtually all of these definitions, it is fair to conclude that there was judicial activism, or at least what New Jersey Governor Chris Christie called “judicial supremacy,” in both of the same sex marriage cases. "