This is quantity of electrical energy which should have a fully charged battery. Capacity is the most important parameter of the battery, because the larger capacity of the battery - the longer it will work without requiring a recharge. Measured capacity in milliamp-hours (mA. per hour). The nominal capacity is always indicated on the label of the battery or the battery itself.
However, actual capacity is not always coincides with the nominal. In practice the actual battery capacity varies from 80% to 110% of the nominal value. In use the capacity of battery changes (usually towards reduction) and depends on various factors: from technology of commissioning and service in use, from conditions and operation term, from used battery chargers, etc.