sideline vs pastime

sideline and pastime both are nouns.

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

As nouns, pastime is a hypernym of sideline; that is, pastime is a word with a broader meaning than sideline:

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

Other hypernyms of sideline include interest, pursuit.

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

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