landmark

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A prominent identifying feature of a landscape.
• noun A fixed marker, such as a concrete block, that indicates a boundary line.
• noun An event marking an important stage of development or a turning point in history.
• noun A building or site with historical significance, especially one marked for preservation by a municipal or national government.
• adjective Having great import or significance.
• transitive verb To accord the status of a landmark to; declare to be a landmark.

from The Century Dictionary.

• noun A boundary-mark to a tract of land; one of the fixed objects used to designate the limits of a farm, town, or other piece of territory, as monumental stones, marked trees, or ditches.
• noun Any specific or prominent object marking a locality or historically associated with a locality; a hill, tree, house, or other feature of a landscape that may serve as a guide; especially, some object on land by which a locality may be recognized by persons at sea.
• noun Figuratively, a distinguishing characteristic, variation, or event; that which marks a turning-point; something that serves to distinguish a particular period of time or point in progress or transition: as, the landmarks of science or history.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun Any conspicuous object on land that serves as a guide; some prominent object, as a hill or steeple.
• noun A structure that has special significance, such as a building with historical associations
• noun An event or accomplishment of great significance. Also used attributively, as a landmark court decision.
• noun important events by which eras or conditions are determined.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A recognizable natural or man-made feature used for navigation.
• noun A notable location with historical, cultural, or geographical significance.
• noun A major, important event.
• verb US To officially designate a site or building as a landmark.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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
• noun an event marking a unique or important historical change of course or one on which important developments depend
• noun the position of a prominent or well-known object in a particular landscape
• noun a mark showing the boundary of a piece of land