An accident is a specific, unexpected, unusual and unintended external action which occurs in a particular time and place, with no apparent and deliberate cause but with marked effects. It implies a generally negative outcome which may have been avoided or prevented had circumstances leading up to the accident been recognized, and acted upon, prior to its occurrence.

Experts in the field of injury prevention avoid use of the term 'accident' to describe events that cause injury in an attempt to highlight the predictable and preventable nature of most injuries. Such incidents are viewed from the perspective of epidemiology - predictable and preventable. Preferred words are more descriptive of the event itself, rather than of its unintended nature (e.g., collision, drowning, fall, etc.)

Accidents of particularly common types (auto, fire, etc.) are investigated to identify how to avoid them in the future. This is sometimes called root cause analysis, but does not generally apply to accidents that cannot be deterministically predicted. A root cause of an uncommon and purely random accident may never be identified, and thus future similar accidents remain "accidental."