by-product vs spin-off

by-product and spin-off both are nouns.

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

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

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

Other synonyms of by-product include byproduct.

by-product (noun)	spin-off (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