darling vs sweetheart

darling and sweetheart both are nouns.

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

As nouns, darling and sweetheart are synonyms defined as:

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

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

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