A landmark was a notable feature of the land or a structure that was easily recognizable and often visible from a distance.
Disorientation could be alleviated by focusing on a landmark. (DS9: "One Little Ship")
In 2375, Neelix studied Earth landmarks while teaching Tom Paris about Talaxian geography. (VOY: "11:59")
It could also refer to an event that separated one period of time from another. In 2371, Julian Bashir noted to Elim Garak that "in many Human cultures, the thirtieth birthday is considered a sort of landmark. It marks the end of youth and the beginning of the slow march into middle age." (DS9: "Distant Voices")