What does "lettuce" mean?

Lettuce: 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.