A nerve gas was an airborne (gaseous or aerosol) agent of chemical warfare. Usually lethal, these compounds worked by inhibiting synaptic activity. Such activity was essential for propagation of of impulses from the central nervous system to other systems of the body; without that activity, the victim became paralyzed and died.
In 2372, when Quark sought to fulfill the terms of a contract with Brunt calling for his death, nerve gas was one option Elim Garak suggested he use to cause it. However, Quark thought the nerve gas smelled bad, and he decided he wanted his death to be sudden. Ultimately, he broke the contract. (DS9: "Body Parts")