lettuce vs iceberg

lettuce and iceberg both are nouns.

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

As nouns, iceberg is a hyponym of lettuce; that is, iceberg is a word with a more specific, narrower meaning than lettuce:

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

lettuce (noun)	iceberg (noun)
leaves of any of various plants of Lactuca sativa	lettuce with crisp tightly packed light-green leaves in a firm head
any of various plants of the genus Lactuca	a large mass of ice floating at sea; usually broken off of a polar glacier
informal terms for money