If there is truly no such thing as an accidental discharge, then there must be no such thing as accidents. Sorry, just a pet peeve of mine. A negligent discharge IS an accidental discharge, since accidents are usually caused by negligence. Saying "it was an accident" doesn't absolve one of responsibility; just ask your auto insurer.
Did you intend to fire the gun? If not, it was an accidental discharge, likely due to either negligence or mechanical failure. Simply put, an ND is a type of AD. I know it sounds good to say that there are no accidents, only negligence. It does help drill it into a person's mind that they are responsible for their own safety in a very serious way, but it's really not technically correct.