An amicus curiae literally means "friend of the court" in Latin. However, in the United States, an amicus curiae brief is an opinon filed by a party with interest in a case's outcome. They are usually written in legal terms and are meant to influence the decision of a court, much like an average brief written by opposing counsels. Amicus briefs are most often filed in cases with a high degree of standing, like Supreme Court Cases, and are usually submitted by third parties, witnesses, or interest groups.
The most notable amicus brief of late is has recently been filed by the Solicitor General for the Bush Administration, in a case involving the affirmative action policy of the University of Michigan. The Administration believes that the policy is unconstitutional under the equal protection clause and therefore must be struck down. It is worthy of mention because it is one of the only ever amicus briefs filed by the Solicitor General, and critics argue that it represents a continuing shift toward the politicization of the judical branch.