romaine lettuce vs lettuce

romaine lettuce and lettuce both are nouns.

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

As nouns, lettuce is a hypernym of romaine lettuce; that is, lettuce is a word with a broader meaning than romaine lettuce:

romaine lettuce: lettuce with long dark-green leaves in a loosely packed elongated head
lettuce: leaves of any of various plants of Lactuca sativa

As nouns, lettuce is a hypernym of romaine lettuce; that is, lettuce is a word with a broader meaning than romaine lettuce:

romaine lettuce (noun)	lettuce (noun)
lettuce with long dark-green leaves in a loosely packed elongated head	leaves of any of various plants of Lactuca sativa
lettuce with long dark-green spoon-shaped leaves	any of various plants of the genus Lactuca
	informal terms for money