An ampere-hour (Ah) is the quantity of charge transported by an electrical current in a given amount of time. It is the current time product at any level of current (Q = I × t). The coulomb is defined as 1 ampere-second, so 1 ampere-hour (Ah) would be 3600 A·s or 3600 C.
Most batteries are rated in Ah. A modern AA alkaline cell is rated at about 2.5–2.8 Ah (at the rate 100h rate). Battery capacities can be very sensitive to discharge rate and the characterizing rate will be given by the manufacturer. Nickel-cadmium and nickel metal-hydride cells, for example, are typically characterized at a 5h rate; sealed lead acid cells are typically characterized at a 20h rate. At different discharge rates, the Ah rating will be different.