behind vs backside

behind and backside both are nouns.

Word	Noun	Adjective	Verb	Adverb
behind	Yes	Yes	No	Yes
backside	Yes	No	No	No

As nouns, behind and backside are synonyms defined as:

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

behind (adjective)	backside (adjective)
having the lower score or lagging position in a contest

behind (adverb)	backside (adverb)
in or to or toward the rear
remaining in a place or condition that has been left or departed from
in debt
in or into an inferior position
showing a time that is earlier than the actual time