Source : Webster's Revised Unabridged Dictionary (1913)

Eggshell \Egg"shell`\, n.
   1. The shell or exterior covering of an egg. Also used
      figuratively for anything resembling an eggshell.

   2. (Zo["o]l.) A smooth, white, marine, gastropod shell of the
      genus {Ovulum}, resembling an egg in form.

Source : WordNet®

eggshell
     n : the exterior covering of a bird's egg [syn: {shell}]