A DIAC is a diode that conducts electrical current only after its breakover voltage has been reached momentarily. It is a two-terminal bidirectional switch that conducts in both directions when the applied voltage exceeds its breakover voltage. The name DIAC comes from the words "Diode AC switch". DIACs are used in the triggering of thyristors, and they are often found in light dimmers such as those used in domestic lighting. They are also widely used in starter circuits for fluorescent lamps.

DIACs have no gate or trigger electrode, unlike some other thyristors that they are commonly used to trigger, such as TRIACs. Some TRIACs, like Quadrac, contain a built-in DIAC in series with the TRIACs gate terminal for this purpose. Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or main terminal MT1 and MT2.

DIACs and SIDACs are often used to deliver a pulse once a capacitor has charged to the breakdown voltage, giving both controlled delay set by the charging resistor and a fixed pulse energy set by the capacitor and breakdown voltage. This is common in simple phase angle controls for AC lamp dimmers and motor speed controls.

In summary, a DIAC is a two-terminal bidirectional switch that conducts in both directions when the applied voltage exceeds its breakover voltage. It is used in the triggering of thyristors and is often found in light dimmers and starter circuits for fluorescent lamps.