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

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "landmark":
   bench mark, bookmark, cairn, cardinal point, catstone, chief thing,
   climax, core, cornerstone, crisis, critical, critical point,
   crucial, crux, essence, essential, essential matter, feature,
   fundamental, gist, gravamen, great point, guide, guidepost, heart,
   high point, historic, identification, important, important thing,
   issue, kernel, keystone, lighthouse, lightship, main point,
   main thing, major, mark, marker, material point, meat, menhir,
   milepost, milestone, momentous, monument, notable, noteworthy, nub,
   pharos, pith, pivot, pivotal, platform, real issue, salient point,
   seamark, significant, sine qua non, substance, substantive point,
   the bottom line, the point, tower, turning point, watchtower,
   watershed



Easton's 1897 Bible Dictionary:

Landmark
   a boundary line indicated by a stone, stake, etc. (Deut. 19:14;
   27:17; Prov. 22:28; 23:10; Job 24:2). Landmarks could not be
   removed without incurring the severe displeasure of God.