At common law, assault is the tort of acting with the intention of causing harmful or offensive contact with another person, or imminent apprehension of such contact, which results in that other person being in put in a state of apprehension.
As distinguished from battery, assault does not involve actual contact, it only needs intent and the resulting apprehension.
While the law varies by jursidiction, contact is often defined as "harmful" if it objectively intends to injure, disfigure, impair, or cause pain. The act is deemed "offensive" if would offend a reasonable person's sense of personal dignity. While "imminence" is judged objectively and varies widely on the facts, it generally suggests there is little to no opportunity for intervening acts. Lastly, the state of "apprehension" should be differentiated from the general state of fear, as apprehension requires only that the person be aware of the imminence of the harmful or offensive act.
A claim of assault can be justified in situations of self-defence or defence of a third party where the act was deemed reasonable. It can also be justified in the context of a sport where consent can often be implied.
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