common lettuce vs lettuce

common lettuce and lettuce both are nouns.

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

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

common lettuce: annual or perennial garden plant having succulent leaves used in salads; widely grown
lettuce: any of various plants of the genus Lactuca

common lettuce (noun)	lettuce (noun)
annual or perennial garden plant having succulent leaves used in salads; widely grown	leaves of any of various plants of Lactuca sativa
	any of various plants of the genus Lactuca
	informal terms for money