Phantom power is a term used in professional audio equipment to describe DC electric power transmitted through microphone cables to operate microphones that contain active electronic circuitry. It is called phantom power because there is no obvious external power supply for the condenser mic; the power supply is invisible and therefore a "phantom". Phantom powering consists of a phantom circuit where direct current is applied equally through the two signal lines of a balanced audio connector, in modern equipment, both pins 2 and 3 of an XLR connector. The supply voltage is referenced to the ground pin of the connector (pin 1 of an XLR), which normally is connected to the cable shield or a ground wire in the cable or both.

Phantom power is used to power microphones that contain active electronic circuitry, such as condenser microphones. When phantom power is turned on, DC current is sent through the XLR cable and delivers the voltage necessary to power the microphone. The power can be provided by a battery located inside of the mic, or most commonly, the DC power is provided by the pre-amp/mixer and delivered to the condenser microphone via the mic cable.

It is important to note that phantom power can cause equipment malfunction or even damage if used with cables or adapters that connect one side of the input to ground, or if certain equipment other than microphones is connected to it. Dynamic microphones, like the SM58, do not require phantom power because they do not have active electronics inside. Nonetheless, applying phantom power will not damage other microphones in the vast majority of cases, as modern dynamic microphones are designed to accept phantom power without issues.