landmark has 2 distinct synonyms spread across 1 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the position of a prominent or well-known object in a particular landscape
“the church steeple provided a convenient landmark”
an event marking a unique or important historical change of course or one on which important developments depend
“the agreement was a watershed in the history of both nations”
Synonyms, antonyms and the broader and narrower relations for landmark come from Princeton University’s WordNet. Full sources and licences.
a mark showing the boundary of a piece of land
an anatomical structure used as a point of origin in locating other anatomical structures (as in surgery) or as point from which measurements can be taken