“”Resolved ... that the State Legislatures are not the proper tribunals to determine the Constitutionality of the laws of the General Government — that the duty of such decision is properly and exclusively confided to the Judicial department.
|—New Hampshire state legislature, 1800|
Judicial "activism" is when a judge or court issues a decision you don't like.
By rhetorical convention, judicial activism occurs when the courts issue rulings that make new law rather than merely explicate and interpret existing law. Judicial activism has also been derided as "legislating from the bench."  There are two fundamental problems with the accusation of judicial activism. First, the basis for much of English Common Law, which serves as the foundation for all law in England and Wales, 49 of 50 states in the United States, eight of ten provinces in Canada, and all of Australia and New Zealand, is the accumulation of rulings made by judges in similar cases in the absence of clear or controlling statutes. Second, every judicial decision requires interpretation of the law and every interpretation makes law.
 Examples of judicial activism
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Furthermore, many studies have shown that conservative judges are far more likely to be "activist" in the sense of overriding elected legislatures. The most "activist" judge on the US Supreme Court has been shown to be Clarence Thomas. 
Arch-conservative Antonin Scalia has been an outspoken opponent of judicial activism. Nevertheless, Mr. Scalia was balls deep in judicial activism when he joined the majority opinion in Medellin v. Texas, which held that despite the rather explicit language of the Constitution, the Vienna Convention on Consular Relations--to which the United States was a party at the time of defendant's arrest--did not apply to Medellin. Whoops!
 Interpretation of the law
The judicial branch's function in government is interpretation of the laws passed by the legislative branch. When a judge's interpretation is seen as liberal, they are labeled an "activist judge". When the interpretation is viewed as conservative, the judge is an originalist or strict-constructionist.
- ↑ Counter-resolutions of Other States in Response to those of Virginia, &c.
- ↑ Daniel Solove, What Exactly Does “Legislating from the Bench” Mean?
- ↑ 463 U.S. 1032 (1983).
- ↑ Id. at 1065-69 (Stevens, J., dissenting).
- ↑ U.S. Const. Art. III § 2 cl. 1. In Long, the Michigan Supreme Court determined that the state's conduct violated the Fourth Amendment of the U.S. Constitution as well as Article 1, § 11 of the Michigan State Constitution. Thus, even if the Michigan Supreme Court's interpretation of the Federal Constitution was incorrect--which the U.S. Supreme Court concluded that it was--the issue is entirely moot as Michigan State law would remain the same. The irony is that this principle notwithstanding, this was not the case--on remand, the Michigan Supreme Court reversed itself.
- ↑ http://www.nytimes.com/2005/07/06/opinion/06gewirtz.html?_r=0
- ↑ 552 U.S. 491 (2008).
- ↑ "This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land..." U.S. Const. Art. VI § 2.
- ↑ Is Justice Scalia Abandoning Originalism?
- ↑ 531 U.S. 98 (2000).