Overview
In common law, assault is the tort of acting intentionally, that is with either general or specific intent, causing the reasonable apprehension of an immediate harmful or offensive contact. Assault requires intent, it is considered an intentional tort, as opposed to a tort of negligence. Actual ability to carry out the apprehended contact is not necessary. 'The conduct forbidden by this tort is an act that threatens violence.'
In criminal law an assault is defined as an attempt to commit battery, requiring the specific intent to cause physical injury.
Comparison to battery
As distinguished from battery, assault does not need to involve the 'unwanted physical contact; but is the anticipation of such contact'. It only needs intent to make or threaten contact and the resulting apprehension. At one point, the common law understanding of assault required more than words alone, it also required an overt act. This understanding has changed, while words alone cannot be construed as assault, words coinciding with actions or circumstances that would put a person in reasonable apprehension that a harm or offensive contact was likely to occur would. For example, an actor shouting "I'm going to kill you" while not moving but in complete shadow and with a knife in their hand could be interpreted as assault.
Additionally, fear is not required for an assault to occur, only anticipation of subsequent battery. A battery can occur without a preceding assault, such as if a person is struck in the back of the head. An assault can be an attempted battery.
From Wikipedia (CC BY-SA 4.0).