From Longman Dictionary of Contemporary Englishlettucelet‧tuce /ˈletɪs/ ●●● S3 noun [countable, uncountable] 
HBPDFFa round vegetable with thin green leaves eaten raw in saladsCOLLOCATIONSadjectivescrisp/crunchy a salad made with crisp lettucelimp/wilted (=lettuce that is not fresh and crisp)The lettuce was all wilted.There was nothing in the fridge except a rather limp lettuce.shredded lettuce (=torn or cut into small pieces)Serve the trout on a bed of shredded lettuce.lettuce + NOUNa lettuce leafTear the lettuce leaves into small pieces.phrasesa head of lettuce (=a complete lettuce)You'll need a head of lettuce for this salad recipe.
HBPDFFa round vegetable with thin green leaves eaten raw in saladsCOLLOCATIONSadjectivescrisp/crunchy a salad made with crisp lettucelimp/wilted (=lettuce that is not fresh and crisp)The lettuce was all wilted.There was nothing in the fridge except a rather limp lettuce.shredded lettuce (=torn or cut into small pieces)Serve the trout on a bed of shredded lettuce.lettuce + NOUNa lettuce leafTear the lettuce leaves into small pieces.phrasesa head of lettuce (=a complete lettuce)You'll need a head of lettuce for this salad recipe.Examples from the Corpus
lettuce• For example, cucumbers, lettuce, tomatoes and tobacco are very susceptible to attacks from the Cucumber Mosaic Virus.• Slice the eggs in half and arrange on a serving dish on a bed of torn frisee lettuce.• The threat is as if a garbage truck had overturned and dumped wilted intellectual lettuce on bystanders.• This comes during a growing season that has been reasonably favorable to California field crops like lettuce.• a head of lettuce• The dressing is great tossed with a salad of lettuce, broccoli florets, zucchini and yellow pepper slices.• Make sure lettuce is available all the time and in sufficient quantity.• Where is the Chicano whose father slaved in the lettuce fields?• Wash the lettuce and tear into pieces.Origin lettuce (1200-1300) Old French laitues, plural of laitue, from Latin lactuca, from lac “milk”; because of its milky juice 