When the earth's crust cracks open and spews hot lava and gases, you've got a volcano on your hands. You better run.
The noun volcano comes from the name of the Roman god of fire, Vulcan. The Romans believed Vulcan had a forge in Mt. Etna, a volcano in Italy. The term can be applied to a vent in a planet's surface or crust. Lava, ash, and gas escape through the vent. The term volcano can also apply to a mountain created by such a vent.