Ampere-hour

The amount of current a power source (such as a battery) can supply for one hour. Batteries are often rated in milliamp hour, a battery rated at 100mAh can supply a circuit 100mA for one hour, or 10mA for 10 hours.

This is helpful in knowing typically how long a battery can power a circuit.

Simply take the rating of the battery (e.g. 250mhA) and divide it by the power requirements for the circuit. The result is the length of time the battery should be able to power the circuit. Know, however, that batteries typically never match their rating exactly, and that batteries may lower in voltage as they discharge (are used). If the voltage drops below the minimum required for the circuit, it may stop powering it even though the battery has not fully drained.