Source : Webster's Revised Unabridged Dictionary (1913)

Acorn \A"corn\, n. [AS. [ae]cern, fr. [ae]cer field, acre; akin
   to D. aker acorn, Ger. ecker, Icel. akarn, Dan. agern, Goth.
   akran fruit, akrs field; -- orig. fruit of the field. See
   {Acre}.]
   1. The fruit of the oak, being an oval nut growing in a woody
      cup or cupule.

   2. (Naut.) A cone-shaped piece of wood on the point of the
      spindle above the vane, on the mast-head.

   3. (Zo["o]l.) See {Acorn-shell}.

Source : WordNet®

acorn
     n : fruit of the oak tree: a smooth thin-walled nut in a woody
         cup-shaped base