A vacuum tube (also called an electron tube) is an electronic device that controls electric current flow in a high vacuum between electrodes, using thermionic emission or other effects. Invented in the early 1900s, they're glass-enclosed components with a heated cathode emitting electrons toward an anode, often with grids for amplification.
They powered early radios, TVs, computers (like ENIAC), and amplifiers until transistors replaced them in the 1950s-60s for being smaller, cheaper, and more reliable. Today, they're used in high-end audio gear, guitar amps, and some scientific instruments.
From web info: Vacuum tubes date back to John Ambrose Fleming's 1904 diode; Lee de Forest added a grid in 1906 for the triode, enabling amplification. Early production by General Electric around 1915 used improved pumps for hard vacuums.