backside vs rearward

backside and rearward both are nouns.

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

As nouns, rearward is a hyponym of backside; that is, rearward is a word with a more specific, narrower meaning than backside:

Other hyponyms of backside include nape, nucha, scruff.

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

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

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