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an electronic valve having three electrodes, a cathode, an anode, and a grid, the potential of the grid controlling the flow of electrons between the cathode and anode. It has been replaced by the transistor

any electronic device, such as a thyratron, having three electrodes

An electron tube used mostly for signal amplification, consisting of a cathode and anode (or plate) as in a diode, and an intervening wire mesh called the control grid. With little voltage on the grid, large currents can flow between the cathode and plate, but small variations in the voltage on the grid cause large variations in this current, allowing for large amplification of a signal applied to the control grid.

See also pentode tetrode