adj. & n.
--adj.
1. happening by chance, unintentionally, or unexpectedly.
2 not essential to a conception; subsidiary.
--n.
1. Mus. a sign indicating a momentary departure from the key signature by raising or lowering a note.
2 something not essential to a conception.
Derivatives:
accidentally adv.
Etymology: ME f. LL accidentalis (as ACCIDENT)