Lettuce was a type of vegetable from Earth.
The galley aboard Enterprise NX-01 featured lettuce, salad, and paprika. (ENT: "The Catwalk")
The acronym "BLT" stood for Bacon, Lettuce, Tomato. It was also B'Elanna Torres' nickname (as well as her initials) while she was at Starfleet Academy. (VOY: "Equinox")
In 2004, Loomis ordered a Double Superbeef at a Burgerland in Detroit without lettuce. (ENT: "Carpenter Street")
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