scarlet letter vs allegory

scarlet letter and allegory both are nouns.

Word	Noun	Adjective	Verb	Adverb
scarlet letter	Yes	No	No	No
allegory	Yes	No	No	No

As nouns, allegory is a hypernym of scarlet letter; that is, allegory is a word with a broader meaning than scarlet letter:

scarlet letter: the letter A in red; Puritans required adulterers to wear it
allegory: a visible symbol representing an abstract idea

Other hypernyms of scarlet letter include emblem.

scarlet letter (noun)	allegory (noun)
the letter A in red; Puritans required adulterers to wear it	a short moral story (often with animal characters)
	an expressive style that uses fictional characters and events to describe some subject by suggestive resemblances; an extended metaphor
	a visible symbol representing an abstract idea