accidental

A property which is not essential; a nonessential; anything happening accidentally.

(painting, plural only) Those fortuitous effects produced by luminous rays falling on certain objects so that some parts stand forth in abnormal brightness and other parts are cast into a deep shadow.

(music) A sharp, flat, or natural, occurring not at the commencement of a piece of music as the signature, but before a particular note.

Part of a text that has a mainly structural purpose, such as spelling, punctuation, or capitalization.