iceberg vs iceberg lettuce

iceberg and iceberg lettuce both are nouns.

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

As nouns, iceberg and iceberg lettuce are synonyms defined as:

iceberg and iceberg lettuce: lettuce with crisp tightly packed light-green leaves in a firm head

Other synonyms of iceberg include crisphead lettuce.

iceberg (noun)	iceberg lettuce (noun)
lettuce with crisp tightly packed light-green leaves in a firm head	lettuce with crisp tightly packed light-green leaves in a firm head
a large mass of ice floating at sea; usually broken off of a polar glacier