foundling vs abandoned infant

foundling and abandoned infant both are nouns.

Word	Noun	Adjective	Verb	Adverb
foundling	Yes	No	No	No
abandoned infant	Yes	No	No	No

As nouns, foundling and abandoned infant are synonyms defined as:

foundling and abandoned infant: a child who has been abandoned and whose parents are unknown

foundling (noun)	abandoned infant (noun)
a child who has been abandoned and whose parents are unknown	a child who has been abandoned and whose parents are unknown