sweetheart vs darling

sweetheart and darling both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweetheart	Yes	Yes	No	No
darling	Yes	Yes	No	No

As nouns, sweetheart and darling are synonyms defined as:

Other synonyms of sweetheart include baby, beloved, dear, dearest, dearie, deary, honey, love, sweetie.

sweetheart (noun)	darling (noun)
a person loved by another person	a special loved one
a very attractive or seductive looking woman	a beloved person; used as terms of endearment
any well-liked individual
a beloved person; used as terms of endearment

sweetheart (adjective)	darling (adjective)
privileged treatment of a favored person or corporation (sometimes unethically)	dearly loved