twin vs identical twin

twin and identical twin both are nouns.

Word	Noun	Adjective	Verb	Adverb
twin	Yes	Yes	Yes	No
identical twin	Yes	No	No	No

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

twin: either of two offspring born at the same time from the same pregnancy
identical twin: either of two twins developed from the same fertilized ovum (having the same genetic material)

Other hyponyms of twin include dizygotic twin, fraternal twin, monozygotic twin, monozygous twin, twimbos.

twin (noun)	identical twin (noun)
a duplicate copy	either of two twins developed from the same fertilized ovum (having the same genetic material)
either of two offspring born at the same time from the same pregnancy

twin (adjective)	identical twin (adjective)
being two identical

twin (verb)	identical twin (verb)
give birth to twins
grow as twins
bring two objects, ideas, or people together
duplicate or match