mAh (milliAmpere-hour)

An ampere-hour or amp-hour (Ah) is a unit of electric charge. Smaller batteries however, such as those in mobile phones and digital cameras, are often rated in milliampere-hours.

The milliampere-hour (mAh) is one-thousandth of an ampere-hour and is a technical term for how much electrical charge a particular battery will hold. 

A battery rated for more mAh will power a phone for a longer amount of time, given the same usage pattern. The trade-off is that batteries with more mAh are generally also physically larger and heavier.

More technically, a higher mAh rating means the (fully-charged) battery can power a device that consumes more power and/or for a longer amount of time before becoming depleted and needing to be re-charged. 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. (In other words, a device using more power will drain the same battery faster.) In that same example, a larger battery, rated 3000 mAh, could power a device drawing 100 milliamps for 30 hours.