incidentally vs by the way

incidentally and by the way both are adverbs.

Word	Noun	Adjective	Verb	Adverb
incidentally	No	No	No	Yes
by the way	No	No	No	Yes

As adverbs, incidentally and by the way are synonyms defined as:

Other synonyms of incidentally include apropos, by the bye.

incidentally (adverb)	by the way (adverb)
introducing a different topic	introducing a different topic
of a minor or subordinate nature