Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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