chewing vs rumination

chewing and rumination both are nouns.

Word	Noun	Adjective	Verb	Adverb
chewing	Yes	No	No	No
rumination	Yes	No	No	No

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

chewing: biting and grinding food in your mouth so it becomes soft enough to swallow
rumination: (of ruminants) chewing (the cud)

Other hyponyms of chewing include chomping, gumming, mumbling.

chewing (noun)	rumination (noun)
biting and grinding food in your mouth so it becomes soft enough to swallow	regurgitation of small amounts of food; seen in some infants after feeding
	(of ruminants) chewing (the cud)
	a calm, lengthy, intent consideration