landmark (n.)

Old English landmearc "object set up to mark the boundaries of a kingdom, estate, etc.," from land (n.) + mearc in its sense "object which marks a boundary or limit" (see mark (n.1)). General sense of "conspicuous object which, by its known position, serves as a guide to a traveler," originally especially an object that can be seen from sea, is from 1560s. Modern figurative sense of "event, etc., considered a high point in history" is from 1859.