pastime vs hobby

pastime and hobby both are nouns.

Word	Noun	Adjective	Verb	Adverb
pastime	Yes	No	No	No
hobby	Yes	No	No	No

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

pastime: a diversion that occupies one's time and thoughts (usually pleasantly)
hobby: an auxiliary activity

Other hyponyms of pastime include avocation, by-line, pursuit, sideline, spare-time activity.

pastime (noun)	hobby (noun)
a diversion that occupies one's time and thoughts (usually pleasantly)	an auxiliary activity
	small Old World falcon formerly trained and flown at small birds
	a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride