The 18650 battery is a rechargeable lithium-ion battery commonly used to power a variety of portable devices such as laptop battery packs, flashlights, cordless tools, and electric vehicles. Its name is derived from its dimensions - the first four digits refer to its physical size, while the fifth digit indicates that it's a cylindrical cell. Typically, the standard 18650 battery measures 18mm in diameter and 65mm in length.

In some cases, modifications have been made to the 18650 battery, including the addition of a button top or an internal protection circuit. These modifications may increase the length of the battery from 65mm to 70mm or even longer, so it's important to confirm the exact specifications required to power a device with the manufacturer.

In summary, the 18650 battery is a versatile and widely-used power source for portable devices, distinguished by its cylindrical shape and specific physical dimensions.