The English word "volcano" descends, by way of Italian and Latin, from Vulcan — the Roman god of fire, forges, and metalworking. The Romans believed Vulcan kept his workshop beneath the Aeolian Islands off the north coast of Sicily, in particular beneath a small island that was always smoking and rumbling. The Romans called the island Vulcano.
One Island, One Word
For most of antiquity, "Vulcano" was a place name — that one specific Sicilian island. The Italian language used it as a common noun by the late Middle Ages: any smoking, rumbling mountain became a vulcano, on the model of the original. English borrowed the Italian word in the late 16th century, with the modern spelling settled by around 1600.
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A Family of Fire Words
Vulcan's name has been productive in English. Vulcanise — the process of treating raw rubber with sulphur to make it stable and elastic — was named in the 19th century in honour of the god of fire and craft, since the process required strong heat. Vulcanian is a technical term in volcanology for a particular kind of explosive eruption. And the science of volcanoes is, of course, volcanology.
The Underworld Workshop
The Roman myth of Vulcan's underground forge was itself a borrowing from Greek, where the same god was called Hephaestus. Greek and Roman thinkers long before modern geology had noticed the connection between volcanic mountains and underground heat. They explained it as a divine workshop. The modern explanation is much more interesting — tectonic plates, magma chambers, and the slow circulation of molten rock — but the Roman name still presides over the entire field.