romaine vs lettuce

romaine and lettuce both are nouns.

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

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

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

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