A DIAC (diode for alternating current) is an electronic component with only two terminals. DIACs are also referred to as symmetrical trigger diodes. The junction between the terminals only becomes conductive after a breakover voltage has been reached.

The DIAC's bidirectional layout allows for the switching of alternating voltages. As soon as the voltage on the terminals (A1 and A2) rises above a certain threshold voltage, it will switch through and the p-n junction becomes conductive. Resistance will rise rapidly again after the current that is flowing through the DIAC falls below a certain value (the holding current.) This procedure is referred to as resetting the DIAC.