An assault is when a person attacks another. In law, it is considered a crime or tort. In common law, it is when somebody threatens or tries to physically hurt someone else. In order to be considered assault, the act must involve the ability to cause the harm. It does not necessarily have to involve touching the other person; that is called battery. In criminal law, an assault can be an attempted but unsuccessful battery. In some states or legal codes, the words "assault" and "battery" may mean the same thing.
There are several different kinds of assault. Each legal system has their own definitions.