A chemical weapon (CW) is a munition that uses chemicals formulated to inflict death or harm on human beings. The Organisation for the Prohibition of Chemical Weapons (OPCW) states:

The term chemical weapon may also be applied to any toxic chemical or its precursor that can cause death, injury, temporary incapacitation or sensory irritation through its chemical action. Munitions or other delivery devices designed to deliver chemical weapons, whether filled or unfilled, are also considered weapons themselves.

They are classified as weapons of mass destruction (WMDs), though they are distinct from nuclear weapons, biological weapons (diseases), and radiological weapons (which use radioactive decay of elements). All may be used in warfare known by the military acronym NBC, for nuclear, biological, and chemical warfare. Weapons of mass destruction are distinct from conventional weapons, which are primarily effective due to their explosive, kinetic, or incendiary potential. Chemical weapons can be widely dispersed in gas, liquid and solid forms, and may easily afflict others than the intended targets. Nerve gas, tear gas and pepper spray are three modern examples.