accidental Definition

1. Happening or done by accident; not planned.

Thesaurus: inadvertent, uncalculated, unintended, unplanned, unexpected, unforeseen, unwitting, chance, fortuitous, serendipitous, random, fluky; Antonym: premeditated, intentional.
2. Incidental; not essential.

1. In written music: a sign, such as a sharp or flat, put in front of a note to show that it is to be played higher or lower than the key signature indicates.
2. Something which is not a necessary feature of something.