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. ... This is when it has come to be generally regarded as settling the law of the question involved.

What makes a Supreme Court decision a landmark decision?

distinguishing a new principle that refines a prior principle, thus departing from prior practice without violating the rule of stare decisis; establishing a test or a measurable standard that can be applied by courts in future decisions.

What was the most important Supreme Court decision?