An early form of triode valve, developed by Lee De Forest and used as a detector and amplifier for radio signals. Frequently attributive, as "audion amplifier", "audion tube", etc.
De Forest discovered that a signal current flowing through a valve could be varied (modulated) by a second current through a wire placed inside the valve in the path of the signal current through it (in the original form of the audion the wire was wrapped round the outside of the glass envelope). The advantage over earlier devices was that the second current did not have to come from the tuned circuit, enabling this to operate more efficiently. It differed from the later form of the triode in that it was not capable of linear amplification.
Early 20th century. From classical Latin audīre to hear + -on.