AC and DC are widely used terms to describe certain behaviour of some physical quantity, such as current, voltage, electric or magnetic field. DC refers to a stable value of a physical quantity, which does not change in time (could be different in space - not changing the DC magnetic field of a magnet has different strength at different distances from the magnet). On the other hand, when some quantity is referred to as AC, it means it is alternating in time. For example, the voltage in electrical sockets is alternating.