'audion' originates from Latin and Greek elements: from Latin 'audire' (the root 'aud-' / 'audio-') meaning 'to hear', combined with the Greek-derived suffix '-on' used to form names of devices or particles.

'audion' was coined in English as the trademarked name 'Audion' by inventor Lee de Forest around 1906 for his triode detector/amplifier. The trade name passed into general English usage to denote early triode vacuum tubes and was later largely replaced by the terms 'triode' or 'vacuum tube'.

Initially it meant the specific device sold under the name 'Audion'; over time it broadened to mean early triode vacuum tubes in general and is now largely archaic or historical in technical contexts.