When conservatives, originalists, constructionists et al aver the term they mean the court has, without legal authority, reshaped and expanded the law in order to give effect to an otherwise unconstitutional policy because the court believes that the otherwise unconstitutional law is sufficiently worthwhile; that is, they replace their subjective opinion of the value of the law with the objective opinion of its legality. Justice Thurgood Marshall once responded to a question at a clerk lunch about his theory of the law with:
However, a judge is to interpret the law not make it. Assertions of activism arise from judicial intrusion upon legislative authority. Belief in the propriety of a law is no basis for its legitimacy. Authority must be found in the constitution.
Liberals mean, near as I can tell, that they disagree with the opinion. Comments following Andrew Sack’s recent NYT article objecting to the Florida ruling revealed consistent themes. The court; doesn’t care about the people, is engaging in cronyism and is denying the rights of the people. Wisdom, wants and wishes may birth motive for a law but they are insufficient to endow it with legitimacy.
Judicial activism does not concern itself with well enough reasoned opinions one disagrees with. The charge is appropriately leveled at when judges exceed their authority and enter the legislative sphere in order to legitimize an otherwise illegitimate law.