back door vs back entrance

back door and back entrance both are nouns.

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

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

Other synonyms of back door include backdoor.

back door (noun)	back entrance (noun)
a secret or underhand means of access (to a place or a position)	an entrance at the rear of 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