romaine lettuce vs romaine

romaine lettuce and romaine both are nouns.

Word	Noun	Adjective	Verb	Adverb
romaine lettuce	Yes	No	No	No
romaine	Yes	No	No	No

As nouns, romaine lettuce and romaine are synonyms defined as:

romaine lettuce and romaine: lettuce with long dark-green leaves in a loosely packed elongated head

Other synonyms of romaine lettuce include cos, cos lettuce.

romaine lettuce (noun)	romaine (noun)
lettuce with long dark-green leaves in a loosely packed elongated head	lettuce with long dark-green leaves in a loosely packed elongated head
lettuce with long dark-green spoon-shaped leaves