Source : Webster's Revised Unabridged Dictionary (1913)

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}.] (Bot.)
   A composite plant of the genus {Lactuca} ({L. 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 {L.
   Canadensis}.

   {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}.

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

Source : WordNet®

sea lettuce
     n : seaweed with edible translucent crinkly green fronds [syn: {laver}]