Source : Webster's Revised Unabridged Dictionary (1913)

Butternut \But"ter*nut`\, n.
   1. (Bot.) An American tree ({Juglans cinerea}) of the Walnut
      family, and its edible fruit; -- so called from the oil
      contained in the latter. Sometimes called {oil nut} and
      {white walnut}.

   2. (Bot.) The nut of the {Caryocar butyrosum} and {C.
      nuciferum}, of S. America; -- called also {Souari nut}.

Source : WordNet®

butternut
     n 1: North American walnut tree having light-brown wood and
          edible nuts; source of a light-brown dye [syn: {butternut
          tree}, {white walnut}, {Juglans cinerea}]
     2: oily egg-shaped nut of an American tree of the walnut family