head lettuce vs lettuce

head lettuce and lettuce both are nouns.

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

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

head lettuce: distinguished by leaves arranged in a dense rosette that develop into a compact ball
lettuce: any of various plants of the genus Lactuca

head lettuce (noun)	lettuce (noun)
distinguished by leaves arranged in a dense rosette that develop into a compact ball	leaves of any of various plants of Lactuca sativa
	any of various plants of the genus Lactuca
	informal terms for money