Source : Webster's Revised Unabridged Dictionary (1913)

Hornwort \Horn"wort`\, n. (Bot.)
   An aquatic plant ({Ceratophyllum}), with finely divided
   leaves.

Source : WordNet®

hornwort
     n 1: any aquatic plant of the genus Ceratophyllum; forms
          submerged masses in ponds and slow-flowing streams
     2: liverworts with slender hornlike 2-valved capsules