Negligent Discharge (ND / AD)
An unintended firing of a gun — 'negligent' when caused by human error, 'accidental' when due to a genuine mechanical failure.
A negligent discharge (ND) is an unintended shot caused by a lapse in safe handling — finger on the trigger too early, careless holstering, mishandling during cleaning, or assuming a gun was unloaded. An accidental discharge (AD) properly refers to a discharge from a true mechanical malfunction, which is rare in modern firearms. In practice the vast majority of unintended shots are NDs.
Nearly every ND is preventable by following the four rules — especially keeping the finger off the trigger and the muzzle pointed in a safe direction, so that even a mistake doesn't cause harm. Extra care during administrative handling (loading, unloading, holstering, cleaning) is where discipline matters most.