romaine vs romaine lettuce

romaine and romaine lettuce both are nouns.

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

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

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

Other synonyms of romaine include cos, cos lettuce.

romaine (noun)	romaine lettuce (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