pastime vs sideline

pastime and sideline both are nouns.

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

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

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

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

pastime (noun)	sideline (noun)
a diversion that occupies one's time and thoughts (usually pleasantly)	an auxiliary activity
	an auxiliary line of merchandise
	a line that marks the side boundary of a playing field

pastime (verb)	sideline (verb)
	remove from the center of activity or attention; place into an inferior position