An accident is a specific, identifiable, unexpected, unusual and unintended external event which occurs in a particular time and place, without apparent or deliberate cause but with marked effects. It implies a generally negative probabilistic outcome which may have been avoided or prevented had circumstances leading up to the accident been recognized, and acted upon, prior to its occurrence.
Narrowly defined, the designation may refer only to the event, while not including the circumstances (facts surrounding) or results of the event; i.e., ‘accident’ is constrained to an immediate incident, the occurrence of which results in an unplanned outcome. In common use, however, ‘accident’ may include the entire interacting circumstantial framework (chance, pre-existing, or uncontrolled dynamically developing conditions; commonplace actions; random time and place; participants; etc.) leading up to, including, and resulting from, the accident's immediate occurrence. Experts in the field of injury prevention avoid use of the term "accident" because they look at these incidents from the perspective of epidemiology - predictable and preventable.
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. For example, a root cause of an uncommon and purely random accident may never be identified, and thus future similar accidents remain "accidental."
Physical examples include, e.g., unintended collisions or falls, being injured by touching something sharp, hot, or electrical, or ingesting poison. Non-physical examples are, e.g, unintentionally revealing a secret or otherwise saying something incorrectly, forgetting an appointment, etc.
The informal term "freak accident" typically refers to an unfortunate and improbable event that seems exceedingly unlikely to happen by chance. In extreme contexts, the term may also imply doubt, ambiguity or suspicion about an accident event's cause.
Colloquially considered negative, 'happy' accidents with positive results are also possible.
The injury prevention community strongly discourages use of the word "accident" to describe events that cause injury in attempt to highlight the predictable and preventable nature of most injuries. Preferred words are more descriptive of the event itself rather than of its unintentional nature (e.g., crash, collision, incident, drowning, fall, etc.)