Source : Webster's Revised Unabridged Dictionary (1913)

Sundial \Sun"di`al\, n.
   An instrument to show the time of day by means of the shadow
   of a gnomon, or style, on a plate.

   {Sundial shell} (Zo["o]l.), any shell of the genus Solarium.
      See {Solarium}.

Source : WordNet®

sundial
     n : timepiece that indicates the daylight hours by the shadow
         that the gnomon casts on a calibrated dial