An assault is the illegal act of causing physical harm or unwanted physical contact to another person, or, in some legal definitions, the threat or attempt to do so. It is both a crime and a tort and, therefore, may result in criminal prosecution, civil liability, or both. Additionally, assault is a criminal act in which a person intentionally causes fear of physical harm or offensive contact to another person. Assault can be committed with or without a weapon and can range from physical violence to threats of violence. Assault is frequently referred to as an attempt to commit battery, which is the deliberate use of physical force against another person. The deliberate inflicting of fear, apprehension, or terror is another definition of assault that can be found in several legal systems. Depending on the severity of the offense, assault may result in a fine, imprisonment, or even death.

Generally, the common law definition is the same in criminal and tort law.

Traditionally, common law legal systems have 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 offenses into a single crime called "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 attempted murder if it was done with intent to kill.