mAh stands for milliampere-hour (milliamp-hour), a measure of how much electric charge a battery can deliver. One mAh is one-thousandth of an ampere-hour, so a 2,000 mAh battery can in theory supply a steady 2,000 milliamps (2 amps) for one hour, or 200 milliamps for about 10 hours, according to TechTarget’s definition.
A higher mAh rating generally means longer runtime for similar batteries in similar devices, but real battery life also depends on how much current the device draws, temperature, and battery age. Because mAh ignores voltage, watt-hours are better for comparing different kinds of batteries: Wh = mAh × volts ÷ 1,000. For example, the 3.7-volt, 2,000 mAh battery pictured stores 7.4 Wh.
Source: TechTarget, “What is milliampere-hour (mAh)?”
Image: Garmin Li-ion battery 2000mAh, by Tiia Monto. CC BY-SA 4.0, via Wikimedia Commons.