The Collaborative International Dictionary of English v.0.48:

Sea lettuce \Sea" let"tuce\ (Bot.)
   The green papery fronds of several seaweeds of the genus
   Ulva, sometimes used as food.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lettuce \Let"tuce\ (l[e^]t"t[i^]s), n. [OE. letuce, prob.
   through Old French from some Late Latin derivative of L.
   lactuca lettuce, which, according to Varro, is fr. lac,
   lactis, milk, on account of the milky white juice which flows
   from it when it is cut: cf. F. laitue. Cf. Lacteal,
   Lactucic.]
   1. (Bot.) A composite plant of the genus Lactuca (Lactuca
      sativa), the leaves of which are used as salad. Plants of
      this genus yield a milky juice, from which lactucarium is
      obtained. The commonest wild lettuce of the United States
      is Lactuca Canadensis.
      [1913 Webster]

   2. United States currency; dollar bills; greenbacks. [slang]
      [PJC]

   Hare's lettuce, Lamb's lettuce. See under Hare, and
      Lamb.

   Lettuce opium. See Lactucarium.

   Sea lettuce, certain papery green seaweeds of the genus
      Ulva.
      [1913 Webster]

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):

sea lettuce
    n 1: seaweed with edible translucent crinkly green fronds [syn:
         sea lettuce, laver]