back entrance vs backdoor

back entrance and backdoor both are nouns.

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

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

Other synonyms of back entrance include back door.

back entrance (noun)	backdoor (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