ICU vs NICU

ICU and NICU both are nouns.

Word	Noun	Adjective	Verb	Adverb
ICU	Yes	No	No	No
NICU	Yes	No	No	No

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

ICU: a hospital unit staffed and equipped to provide intensive care
NICU: an intensive care unit designed with special equipment to care for premature or seriously ill newborn

Other hyponyms of ICU include coronary care unit, neonatal intensive care unit.

ICU (noun)	NICU (noun)
a hospital unit staffed and equipped to provide intensive care	an intensive care unit designed with special equipment to care for premature or seriously ill newborn