The Collaborative International Dictionary of English v.0.48:

Cairn \Cairn\, n. [Gael. carn, gen. cairn, a heap: cf. Ir. & W.
   carn.]
   1. A rounded or conical heap of stones erected by early
      inhabitants of the British Isles, apparently as a
      sepulchral monument.
      [1913 Webster]

            Now here let us place the gray stone of her cairn.
                                                  --Campbell.
      [1913 Webster]

   2. A pile of stones heaped up as a landmark, or to arrest
      attention, as in surveying, or in leaving traces of an
      exploring party, etc. --C. Kingsley. Kane.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cairn
    n 1: a mound of stones piled up as a memorial or to mark a
         boundary or path
    2: small rough-haired breed of terrier from Scotland [syn:
       cairn, cairn terrier]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "cairn":
   arch, barrow, bench mark, bookmark, boundary stone, brass, bust,
   catstone, cenotaph, column, cromlech, cross, cup, cyclolith,
   dolmen, footstone, grave, gravestone, headstone, hoarstone,
   inscription, landmark, lighthouse, lightship, mark, marker,
   mausoleum, megalith, memento, memorial, memorial arch,
   memorial column, memorial statue, memorial stone, menhir, milepost,
   milestone, monolith, monument, mound, necrology, obelisk, obituary,
   pharos, pillar, plaque, platform, prize, pyramid, reliquary,
   remembrance, ribbon, rostral column, seamark, shaft, shrine, stela,
   stone, stupa, tablet, testimonial, tomb, tombstone, tope, tower,
   trophy, watchtower