[1] : The Collaborative International Dictionary of English v.0.48

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.
      1913 webster

   2. any conspicuous object on land that serves as a guide;
      some prominent object, as a hill or steeple.
      1913 webster

   3. a structure that has special significance, such as a
      building with historical associations; especially, a
      building that is protected from destruction or alteration
      by special laws intended to preserve structures of
      historical significance; as, a landmark preservation law.
      pjc

   4. an event or accomplishment of great significance; as,
      brown v. board of education was a landmark of the civil
      rights movement. also used attributively, as a landmark
      court decision.
      pjc

   landmarks of history, important events by which eras or
      conditions are determined.
      1913 webster
see also:
land mark landmarks of history 

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