heir vs heiress

heir and heiress both are nouns.

Word	Noun	Adjective	Verb	Adverb
heir	Yes	No	No	No
heiress	Yes	No	No	No

As nouns, heiress is a hyponym of heir; that is, heiress is a word with a more specific, narrower meaning than heir:

heir: a person who is entitled by law or by the terms of a will to inherit the estate of another
heiress: a female heir

Other hyponyms of heir include heir apparent, heir-at-law, inheritress, inheritrix, heir presumptive.

heir (noun)	heiress (noun)
a person who is entitled by law or by the terms of a will to inherit the estate of another	a female heir
a person who inherits some title or office