Triode
The tube that could amplify: a third electrode letting a weak voltage steer a large current, which is what made electronics active.
In 1906 Lee de Forest slipped a third electrode — a control grid — between the filament and plate of Fleming's diode and called the result the Audion. The grid let a weak signal govern a strong current with no moving parts: amplification and fast switching in one part. De Forest barely understood why it worked; Edwin Armstrong explained the physics a few years later.
Radio came first, computers second. The vacuum tube used as a logic gate was always a triode or its cousin the pentode — ENIAC held around 18,000, Colossus a couple of thousand. Each switched perhaps a thousand times faster than a relay, but each also burned out, which set the practical ceiling on how large a tube machine could grow. The transistor (1947) did the same job cold and removed the ceiling.