A vacuum tube is an electronic device that controls the flow of electric current in a vacuum. It consists of electrodes sealed in a glass or metal envelope from which air has been removed. When a voltage is applied, electrons can move between the electrodes, allowing the tube to amplify signals or switch currents. Vacuum tubes were widely used in early radios, televisions, and computers before the invention of transistors. They played a crucial role in the development of electronic technology throughout the 20th century, enabling advancements in communication and entertainment.