back door vs exterior door

back door and exterior door both are nouns.

Word	Noun	Adjective	Verb	Adverb
back door	Yes	No	No	No
exterior door	Yes	No	No	No

As nouns, exterior door is a hypernym of back door; that is, exterior door is a word with a broader meaning than back door:

Other hypernyms of back door include outside door.

back door (noun)	exterior door (noun)
a secret or underhand means of access (to a place or a position)	a doorway that allows entrance to or exit from a building
an entrance at the rear of a building
an undocumented way to get access to a computer system or the data it contains