back end vs backside

back end and backside both are nouns.

Word	Noun	Adjective	Verb	Adverb
back end	Yes	No	No	No
backside	Yes	No	No	No

As nouns, back end and backside are synonyms defined as:

Other synonyms of back end include rear.

back end (noun)	backside (noun)
the side of an object that is opposite its front	the fleshy part of the human body that you sit on
	the side of an object that is opposite its front