Source : Webster's Revised Unabridged Dictionary (1913)

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).

Source : WordNet®

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