The crime of assault varies from state to state (and under federal law), but is generally defined as intentionally putting another person in reasonable apprehension or fear of an imminent (immediate) harmful or offensive contact. The crime of assault may be committed even if there is no physical contact, as the criminal physical contact constitutes the separate crime of battery.

The crime of assault with a deadly weapon (also known as aggravated assault) occurs when someone uses a deadly weapon (gun, knife, club, tire iron, brass knuckles, etc.) during an assault.

In Illinois, assault is defined under 720 ILCS 5/12-1 as an act where an individual knowingly engages in conduct which places another person in reasonable apprehension of receiving a battery. This means that the victim must believe that they are about to be physically harmed or touched in an offensive way, even if no physical contact actually occurs. Aggravated assault, as per 720 ILCS 5/12-2, occurs when the assault includes aggravating factors such as the use of a deadly weapon, wearing a hood or mask to conceal identity, or the assault being committed against certain protected classes of people (e.g., teachers, the elderly, handicapped, etc.). The use of a deadly weapon like a gun, knife, or other objects that could cause serious harm elevates the crime to aggravated assault, which carries more severe penalties than simple assault. Both assault and aggravated assault are criminal offenses and are subject to prosecution under Illinois state law.