romaine lettuce vs cos

romaine lettuce and cos both are nouns.

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

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

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

Other synonyms of romaine lettuce include cos lettuce, romaine.

romaine lettuce (noun)	cos (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	ratio of the adjacent side to the hypotenuse of a right-angled triangle