spin-off vs by-product

spin-off and by-product both are nouns.

Word	Noun	Adjective	Verb	Adverb
spin-off	Yes	No	No	No
by-product	Yes	No	No	No

As nouns, spin-off and by-product are synonyms defined as:

spin-off and by-product: a product made during the manufacture of something else

Other synonyms of spin-off include byproduct.

spin-off (noun)	by-product (noun)
a product made during the manufacture of something else	a product made during the manufacture of something else
	a secondary and sometimes unexpected consequence