A Landmark decision, or Landmark court decision, establishes new precedents that establish a significant new legal principle or concept. Or it changes the interpretation of existing law. In Commonwealth countries, a reported decision is said to be a leading decision. This is when it has come to be generally regarded as settling the law of the question involved.
A landmark decision is "a most important case which has establish a law firmly in an area, usually referring to a U.S. Supreme Court case." A landmark decision may have either long-term or short-term significance. Politics, economics or other changes in society may reduce the effects of a landmark decision. A landmark decision is one that changes an entire area of the law during a period of time.
The United States Constitution did not provide for judicial review of laws and court decisions. It was a power the US Supreme court assumed (took) for itself with its first landmark decision. In the decision Marbury v. Madison (1803) the court established its "power to say what the law is". The court gave itself the right to interpret the constitution.