Source : Webster's Revised Unabridged Dictionary (1913)

Landmark \Land"mark`\, n. [AS. landmearc. See {Land}, and {Mark}
   a sign.]
   1. A mark to designate the boundary of land; any, mark or
      fixed object (as a marked tree, a stone, a ditch, or a
      heap of stones) by which the limits of a farm, a town, or
      other portion of territory may be known and preserved.

   2. Any conspicuous object on land that serves as a guide;
      some prominent object, as a hill or steeple.

   {Landmarks of history}, important events by which eras or
      conditions are determined.

Source : WordNet®

landmark
     n 1: the position of a prominent or well-known object in a
          particular landscape; "the church steeple provided a
          convenient landmark"
     2: 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"
        [syn: {turning point}, {watershed}]
     3: a mark showing the boundary of a piece of land
     4: 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