n.

1. a musical notation that makes a note sharp or flat or natural although that is not part of the key signature

adj.

1. associated by chance and not an integral part;
   adventitious
   
   • poetry is something to which words are the accidental, not by any means the essential form"- Frederick W. Robertson; "they had to decide whether his misconduct was adventitious or the result of a flaw in his character
2. occurring or appearing or singled out by chance;
   casual, chance(a)
   
   • their accidental meeting led to a renewal of their friendship
   • seek help from casual passers-by
   • a casual meeting
   • a chance occurrence
3. without intention (especially resulting from heedless action);
   inadvertent
   
   • with an inadvertent gesture she swept the vase off the table
   • accidental poisoning
   • an accidental shooting