accidental

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

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.

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

Not essential; incidental, secondary.

Nonessential to something's inherent nature (especially in Aristotelian thought).

Adjusted by one or two semitones, in temporary departure from the key signature.