iceberg lettuce vs head lettuce

iceberg lettuce and head lettuce both are nouns.

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

iceberg lettuce (noun)	head lettuce (noun)
lettuce with crisp tightly packed light-green leaves in a firm head	distinguished by leaves arranged in a dense rosette that develop into a compact ball