back entrance vs back door

back entrance and back door both are nouns.

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

As nouns, back entrance and back door are synonyms defined as:

Other synonyms of back entrance include backdoor.

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