1 An unintended, unforeseen, and undesirable event, especially one that causes harm, injury, damage, or loss. 2An unintended and unexpected event, especially one that is undesirable or harmful, that does not occur in the usual course of events under the circumstances in which it occurred, or that would not be reasonably anticipated. 3 In equity, an unexpected and injurious event not caused by misconduct, mistake, or negligence. 4 In many automobile insurance policies, any unintentional event ...