rearward vs backside

rearward and backside both are nouns.

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

As nouns, backside is a hypernym of rearward; that is, backside is a word with a broader meaning than rearward:

Other hypernyms of rearward include back end, rear.

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

rearward (adjective)	backside (adjective)
located in or toward the back or rear
directed or moving toward the rear

rearward (adverb)	backside (adverb)
at or to or toward the back or rear