iceberg lettuce vs lettuce

iceberg lettuce and lettuce both are nouns.

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

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

iceberg lettuce: lettuce with crisp tightly packed light-green leaves in a firm head
lettuce: leaves of any of various plants of Lactuca sativa

iceberg lettuce (noun)	lettuce (noun)
lettuce with crisp tightly packed light-green leaves in a firm head	leaves of any of various plants of Lactuca sativa
	any of various plants of the genus Lactuca
	informal terms for money