Introducing a different topic:
— Incidentally, I won't go to the party.

synonyms: apropos, by the bye, by the way.

Roget 10: parenthetically etc. adj.; by the way, by the by; en passant [Fr.], incidentally; irrespectively etc. adj.; without reference to, without regard to; in the abstract etc. 87; a se.    ... show more

Roget 134: opportunely etc. adj.; in proper course, in due course, in proper season, in due season, in proper time, in due time; for the nonce; in the nick of time, in the fullness of time; all in good time; just in time, at the eleventh hour, now or never.    by the way, by the by; en passant [Fr.], a propos [Fr.]; pro re nata [Lat.], pro hac vice [Lat.]; par parenthese [Fr.], parenthetically, by way of parenthesis, incidentally; while speaking of, while on the subject; extempore; on the spur of the moment, on the spur of the occasion; on the spot etc. (early) 132.    ... show more

Roget 621: randomly, by chance, fortuitously; unpredictably, unforeseeably; casually etc. 156; unintentionally etc. adj.; unwittingly.    en passant [Fr.], by the way, incidentally; as it may happen; at random, at a venture, at haphazard.    ... show more

Of a minor or subordinate nature:
— These magnificent achievements were only incidentally influenced by Oriental models.

synonym: accidentally.