Here is the US government definition:
Chemical weapon. — The term "chemical weapon" means the following, together or separately:
(A) A toxic chemical and its precursors, except where intended for a purpose not prohibited under this Act as long as the type and quantity is consistent with such a purpose.
(B) Ammunition or device, specifically designed to cause death or other harm through toxic properties of those toxic chemicals specified in subparagraph (A), which would be released as a result of the employment of such munition or device.
(C) Any equipment specifically designed for use directly in connection with the employment of munitions or devices specified in subparagraph (B).