Incidentally means by the way, said when adding a side point. Incidentally, he was there too, mentioned incidentally.
It signals that what follows is off the main track, and it is very often used to introduce the most interesting thing in the sentence. It also has a plain sense meaning by chance rather than by design, as in a discovery made incidentally.
1adverbincidentallyBy the way, said when adding a side point.
“Incidentally, have you seen my blue umbrella anywhere?”
incidentally is a connective.
From incidental + -ly.
Etymology from Wiktionary, CC BY-SA