accidental

Concise Oxford English Dictionary © 2008 Oxford University Press:
accidental/aksɪˈdentl/

▶adjective

• 1 happening by accident.

• 2 incidental.

• 3 Philosophy relating to or denoting properties not essential to a thing's nature.

▶noun

• 1 Music a sign indicating a momentary departure from the key signature by raising or lowering a note.

• 2 Ornithology another term for vagrant.

– derivatives
accidentally adverb.