A tetrode is an electronic device having four active electrode

Electrode

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

s. The term most commonly applies to a two-grid vacuum tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

. It has the three electrodes of a triode

Triode

A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...

 and an additional screen grid which significantly changes its behaviour.
The grid nearest the cathode is the "control grid

Control grid

The control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...

"; the voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 applied to it causes the anode

Anode

An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 current to vary. In normal operation, with a resistive load, this varying current will result in varying (AC) voltage measured at the anode

Anode

An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

.