AC may refer to any of the following:
1. Short for Alternating Current, AC is an electrical current that reverses or alternates its direction of flow. On May 1, 1888, Nikola Tesla patented the rotating field motor, an invention that generates and transmits AC power, still in use today. Tesla later sold the rights to George Westinghouse.
AC power's main advantage over DC is that it's voltage can be changed using a device called a transformer. A standard wall outlet in the United States is 110 volts transmitting currents at 60 hertz per second. European power systems use 220 volts alternating current at 50 Hz.
When plugged into an AC power outlet, your computer's power supply changes this "dirty power", converting it into direct current. It also drops the voltage to keep the sensitive components in your computer protected.
2. Short for Anonymous Coward, AC is commonly used in message boards and newsgroups for users who do not want to reveal their identity.
3. A Linux command. See the ac command page for further information.