An assault is the act of inflicting physical harm or unwanted physical contact upon a person or, in some specific legal definitions, a threat or attempt to commit such an action. It is both a crime and a tort and, therefore, may result in criminal prosecution, civil liability, or both. Generally, the common law definition is the same in criminal and tort law.

Traditionally, common law legal systems had separate definitions for assault and battery. When this distinction is observed, battery refers to the actual bodily contact, whereas assault refers to a credible threat or attempt to cause battery. Some jurisdictions combined the two offences into assault and battery, which then became widely referred to as "assault". The result is that in many of these jurisdictions, assault has taken on a definition that is more in line with the traditional definition of battery. The legal systems of civil law and Scots law have never distinguished assault from battery.

Legal systems generally acknowledge that assaults can vary greatly in severity. In the United States, an assault can be charged as either a misdemeanor or a felony. In England and Wales and Australia, it can be charged as either common assault, assault occasioning actual bodily harm (ABH) or grievous bodily harm (GBH). Canada also has a three-tier system: assault, assault causing bodily harm and aggravated assault. Separate charges typically exist for sexual assaults, affray and assaulting a police officer. Assault may overlap with an attempted crime; for example an assault may be charged as an attempted murder if it was done with intent to kill.