A measure of charge (or current flow over time).
One ampere-hour (or amp-hour or Ah) is a current of one ampere flowing for one hour. The amount of charge transferred in that hour is 3,600 coulombs (ampere-seconds).
A milliampere-hour (mAh or milliamp-hour) is a thousandth of an amp-hour.
An ampere-second (A-s or amp-second) is an amp supplied for one second.
A common use of the term is rating energy storage device capacity, especially rechargeable batteries. For example, a 12-volt, 7Ah rechargeable battery used in an alarm system will supply an amp at the rated voltage range for seven hours, 2 amps for 3.5 hours, etc. If my alarm consumes 250mA, this battery would operate the system for 28 hours.
