Ah Ampere-hours The capacity rating of an electrical cell or battery.
One point to watch when interpreting ampere-hour ratings is that the capacity of a battery or cell varies depending on the rate of discharge. The figure is normally specified at the 10-hour rate. E.g. A 10Ah battery will deliver a current of 1 amp for 10 hours. It will however, deliver a current of 10 amps for somewhat less than 1 hour. I.e. the effective capacity is reduced at high discharge rates (and also, not greatly increased at very low discharge rates).
In combination with the average battery terminal voltage throughout the discharge cycle, the battery capacity is also a measure of the available stored energy. E.g. a 6V 10Ah battery can deliver 6 Watt (6 Volt x 1 Amp ) for 10 hours, i.e. 6 Watts for 36000 seconds = 216000 Joule (216KJ).
Some modern batteries have energy densities close to that of Dynamite, and pose a severe fire hazard if accidentally short-circuited.