A chemical weapon is an device which deploys deadly chemicals to a target. A common chemical weapon used in ancient Earth wars was mustard gas.
The designs for the chemical weapons were present in the databanks on the USS Verdun. In 2163 Commodore Lucian Murat had downloaded the designs that were used by the Human forces on Torgu-Va to make this weapon in their fight against the Tarn. Both Tarn and Human forces used mortars and airplanes to deliver the chemical weapons. (TNG novel: The Forgotten War)
During the Vaadwaur Supremacy invasion of Kobali Prime in 2409-2410, the Vaadwaur developed and deployed a chemical weapon that targeted Kobali exclusively. The Alpha Quadrant Alliance, which had entered the war on the side of the Kobali, was forced to assault the Vaadwaurs' forward operating base to destroy the stocks of gas. (STO - "The Kobali Front" missions: "Looming Shadows", "Hidden Assault", "With Our Last Breath")