Source : Webster's Revised Unabridged Dictionary (1913)

Alternating current \Al"ter*nat`ing cur"rent\ (Elec.)
   A current which periodically changes or reverses its
   direction of flow.

Source : WordNet®

alternating current
     n : an electric current that reverses direction sinusoidally;
         "In the US most household current is AC at 60 cycles per
         second" [syn: {AC}] [ant: {direct current}]