Definition of Lettuce: /ˈlɛtɪs/
noun: lettuce; plural noun: lettuces
a cultivated plant of the daisy family, with edible leaves that are eaten in salads.
used in names of other plants with edible green leaves, e.g. lamb's lettuce, sea lettuce.

Origin: Middle English: from Old French letues, laitues, plural of laitue, from Latin lactuca, from lac, lact- ‘milk’ (because of its milky juice).