noun

• 1A chlorine oxide (dichlorine monoxide), Cl₂ O, an unstable yellow or orangish gas, used in chemical synthesis as a chlorinating and oxidizing agent (also called hypochlorous anhydride) (now rare).

• 2A free radical, ClO·, formed especially in the upper atmosphere by the action of ultraviolet light on chlorofluorocarbons (CFCs) and contributing to ozone depletion.