NICU vs intensive care unit

NICU and intensive care unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
NICU	Yes	No	No	No
intensive care unit	Yes	No	No	No

As nouns, intensive care unit is a hypernym of NICU; that is, intensive care unit is a word with a broader meaning than NICU:

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

Other hypernyms of NICU include ICU.

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