Volcanic gas or volcanic vapor was gas associated with volcanic activity.
Such vapor could be inhaled as a drug, leaving an aftertaste of sulfur and ash. In 2257, Sylvia Tilly was invited by an Orion at the Orion outpost on Qo'noS to partake of volcanic vapor, "straight from the source" (the planet's underground volcano system). Though she tried to refuse, he insisted and she passed out from the experience. When she woke up, she found the Orion trying to steal the case attached to her wrist. Tilly realized that the vapor meant the volcanoes were live, not extinct as Starfleet believed. (DIS: "Will You Take My Hand?")
In 2367, Lieutenant Geordi La Forge reported high levels of volcanic gases inside a cave on Nelvana III, including sulfur, methane, and hydrogen sulfide. (TNG: "Future Imperfect")
In 2370, Odo experienced a metamorphic reaction to volcanic gas on L-S VI, causing him to transform into an aggressive creature. (DS9: "The Alternate")