Let·tuce n.
 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.
 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.
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 Sea let·tuce Bot. The green papery fronds of several seaweeds of the genus Ulva, sometimes used as food.
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 Ul·va prop. n.  Bot. A genus of thin papery bright green seaweeds including the kinds called sea lettuce.
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 sea lettuce
      n : seaweed with edible translucent crinkly green fronds [syn: laver]