Phantom Power
From WikiRecording
Phantom Power is a method of supplying power to devices over a standard XLR cable. It's most often used for condenser microphones, but other types of devices, such as direct boxes, may also use phantom power. It is called "phantom power" because it should be invisible to devices that do not use it, however some types of microphones can be damaged by it (see Precautions below).
Phantom power is most often provided by mixers and preamps. Devices that supply phantom power will typically have a switch that allows you to disable it, either on individual channels, in banks, or for the entire channel set.
Precautions
Most modern microphones will be compatible with phantom power, and those that do not require phantom power (such as dynamic microphones) will ignore it. However, some older microphones (ribbons in particular) can be damaged if subjected to phantom power. Some Vintage synths and keyboards can also be severely damaged by phantom power.
References
Wikipedia's article on phantom power
