NOUN (2)

1. any aquatic plant of the genus Ceratophyllum; forms submerged masses in ponds and slow-flowing streams;

2. liverworts with slender hornlike capsules;

The Collaborative International Dictionary of English v.0.48:

Hornwort \Horn"wort`\, n. (Bot.)
   An aquatic plant (Ceratophyllum), with finely divided
   leaves.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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