An unintended discharge may be produced by an incompatibility between firearm design and usage, such as the phenomenon of cooking off a round in a closed bolt machine gun, a mechanical malfunction as in the case of slamfire in an automatic weapon, or be user induced due to training issues or negligence.
The phenomenon has also been defined in scientific literature as an activation of the trigger mechanism that results in an unplanned discharge that is outside of the firearm’s prescribed use, where "prescribed use" refers to departmental policies and laws related to the operation of firearms (O'Neill, 2018).
This can include things like firearms that do not have mechanisms to render them drop safe falling a sufficient distance,[1] a firing pin stuck forward,[2] a sear failing,[3] or rounds heating sufficiently to spontaneously ignite in the chamber (as may happen in a closed bolt machine gun).
In judicial and military technical terms, a negligent discharge is a chargeable offence.
From an article on a U.S. Air Force website: A negligent discharge occurs when a weapon is fired due to either operator error or a lack of attention to basic safety rules.