back door vs outside door

back door and outside door both are nouns.

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

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

Other hypernyms of back door include exterior door.

back door (noun)	outside 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