premature infant vs preemie

premature infant and preemie both are nouns.

Word	Noun	Adjective	Verb	Adverb
premature infant	Yes	No	No	No
preemie	Yes	No	No	No

As nouns, premature infant and preemie are synonyms defined as:

premature infant and preemie: an infant that is born prior to 37 weeks of gestation

Other synonyms of premature infant include premature baby, premie, preterm baby, preterm infant.

premature infant (noun)	preemie (noun)
an infant that is born prior to 37 weeks of gestation	an infant that is born prior to 37 weeks of gestation