The Collaborative International Dictionary of English v.0.48:

Water lettuce \Wa"ter let"tuce\ (Bot.)
   A plant (Pistia stratiotes) which floats on tropical
   waters, and forms a rosette of spongy, wedge-shaped leaves.
   --J. Smith (Dict. Econ. Plants).
   [1913 Webster]

WordNet (r) 3.0 (2006):

water lettuce
    n 1: pantropical floating plant forming a rosette of wedge-
         shaped leaves; a widespread weed in rivers and lakes [syn:
         pistia, water lettuce, water cabbage, Pistia
         stratiotes, Pistia stratoites]