The word gas was first used by the early 17th-century Flemish chemist Jan Baptist van Helmont. He identified carbon dioxide, the first known gas other than air. Van Helmonts word appears to have been simply a phonetic transcription of the Ancient Greek word   – the g in Dutch being pronounced like ch in "loch" (voiceless velar fricative, ) – in which case Van Helmont simply was following the established alchemical usage first attested in the works of Paracelsus. According to Paracelsuss terminology, chaos meant something like .
An alternative story is that Van Helmonts term was derived from "gahst (or geist), which signifies a ghost or spirit". That story is given no credence by the editors of the Oxford English Dictionary'. In contrast, the French-American historian Jacques Barzun speculated that Van Helmont had borrowed the word from the German , meaning the froth resulting from fermentation.