1 a : an object or structure on land that is easy to see and recognize
The Golden Gate Bridge is a famous/familiar landmark in San Francisco.
1 b US : a building or place that was important in history
The battlefield is a national historical landmark.
2 : a very important event or achievement
The moon landing is a landmark in space exploration.
The decision/ruling was a landmark in legal history.
— often used before another noun
a landmark legal case
a landmark court decision
a landmark discovery