in
photography,
PC stands for
Prontor-
Compur (two brands of
shutters), and it identifies that little
coaxial connector that you will find on most old cameras and on all
professional cameras and
studio flashes.
It is usually found on a PC cable, and its function is to
synchronize the
flash with the
shutter, that's to say, to fire the
flash when the
shutter is fully open.
It is very old technology. Basically, you connect the flash and the camera with PC cable, and when the shutter opens the camera short-circuits the two wires, and the flash goes off.
In some old flash systems you will find quite a robust voltage on the PC connector, so don't put it in your mouth.