1.a musical notation that makes a note sharp or flat or natural although that is not part of the key signature
2.associated by chance and not an integral part;
poetry is something to which words are the accidental, not by any means the essential form- Frederick W. Robertson
3.occurring or appearing or singled out by chance;
their accidental meeting led to a renewal of their friendship
4.without intention (especially resulting from heedless action);
with an inadvertent gesture she swept the vase off the table