triode

An electron tube used mostly for signal amplification, consisting of a cathode, an anode, and an intervening wire mesh called a control grid.

A thermionic valve containing an anode, a cathode, and a control grid; small changes to the charge on the grid control the flow from cathode to anode which makes amplification possible

a thermionic vacuum tube having three electrodes; fluctuations of the charge on the grid control the flow from cathode to anode which makes amplification possible