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]

WordNet (r) 3.0 (2006):

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: landmark, 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