opening in or near a volcano, through which hot sulphurous gases emerge

A fumarole (Latin fumus, smoke) is an opening in the crust of the Earth, often in the neighborhood of dormant volcanoes, where steam and gases come out, for instance carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide.

Fumaroles near the summit on Mount Damavand, Iran
Sulfureous fumaroles, Whakaari/White Island, New Zealand
Fumaroles in Nisyros, Greece

Etymology change

The name solfatara (from the Italian solfo, sulfur), is given to fumaroles with sulfurous gases.