means A recognizable feature of a landscape—or a building, monument, or moment—prominent enough to guide your way or mark a turning point in history.

from A plain old English compound of "land" plus "mark," meaning literally a mark set on the land. The earliest sense was strictly practical: a boundary marker, a stone or post that told you where one person's field ended and another's began. Only later did it widen to any conspicuous object you steer by, and later still to figurative "milestones" in time. Both halves are ancient Germanic roots—"mark" is a cousin of words for boundary and sign across the Germanic languages—so this is one of those honest, home-grown words with no exotic story behind it, just a stone in a field doing its job for a thousand years.