1) ADV: ADV with cl You use incidentally to introduce a point which is not directly relevant to what you are saying, often a question or extra information that you have just thought of.

`I didn't ask you to come. Incidentally, why have you come?'...

The tower, incidentally, dates from the twelfth century.

by the way
2) ADV: ADV with v If something occurs only incidentally, it is less important than another thing or is not a major part of it.

The letter mentioned my great-aunt and uncle only incidentally.

English dictionary. 2008.