1.AdverbIn an incidental manner.
By happenstance; in a particular case which has little to no bearing on, and little to no causal relation to, any other case.
"The book discussed the subject, but only incidentally.; Incidentally, did you hear anything new from your brother yesterday?"
2.AdverbBy happenstance; in a particular case which has little to no bearing on, and little to no causal relation to, any other case.
3.AdverbNot of central or critical importance.
"The book discussed the subject, but only incidentally."
4.AdverbParenthetically, by the way.
"Incidentally, did you hear anything new from your brother yesterday?"