AC0 is a complexity class used in circuit complexity. It is the smallest class in the AC hierarchy, and consists of all circuits of depth O(1) and polynomial size, with unlimited-fanin AND gates and OR gates. (We allow NOT gates only at the inputs). It thus contains NC0, which has only bounded-fanin AND and OR gates.
From a descriptive complexity viewpoint, DLOGTIME-uniform AC0 is equal to the descriptive class FO+BIT of all languages describable in first-order logic with the addition of the BIT operator, or alternatively by FO(+, ).