Milliamp Hours. A unit for measuring electric power over time. mAh is commonly used to describe the total amount of energy a battery can store.
A higher mAh rating means the battery can power a device that comsumes more power and/or for a longer amount of time. For example, a battery rated at 1500 mAh can power a device drawing 100 milliamps for 15 hours, or a device drawing 150 milliamps for 10 hours.