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]