adjective

Happening by chance, unintentionally, or unexpectedly.

a verdict of accidental death

fortuitous,chance,occurring by accident,occurring by chance,adventitious,fluky,coincidental,casual,serendipitous,random,aleatory

noun

A sign indicating a momentary departure from the key signature by raising or lowering a note.

horn parts are usually written without key signature, the necessary accidentals being added