The Collaborative International Dictionary of English v.0.48:

Heirloom \Heir"loom`\, n. [Heir + loom, in its earlier sense of
   implement, tool. See Loom the frame.]
   Any furniture, movable, or personal chattel, which by law or
   special custom descends to the heir along with the
   inheritance; any piece of personal property that has been in
   a family for several generations.
   [1913 Webster]

         Woe to him whose daring hand profanes
         The honored heirlooms of his ancestors.  --Moir.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heirloom
    n 1: (law) any property that is considered by law or custom as
         inseparable from an inheritance is inherited with that
         inheritance
    2: something that has been in a family for generations

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "heirloom":
   bequeathal, bequest, birthright, borough-English, coheirship,
   coparcenary, entail, gavelkind, heirship, hereditament, heritable,
   heritage, heritance, incorporeal hereditament, inheritance,
   law of succession, legacy, line of succession, mode of succession,
   patrimony, postremogeniture, primogeniture, reversion, succession,
   ultimogeniture