The Collaborative International Dictionary of English v.0.48:

Ulva \Ul"va\, prop. n. [L., sedge.] (Bot.)
   A genus of thin papery bright green seaweeds including the
   kinds called sea lettuce.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Ulva
    n 1: type genus of the family Ulvaceae; green seaweed having a
         thallus two cells thick: sea lettuce [syn: Ulva, genus
         Ulva]