An 26650 Battery is a lithium-ion rechargeable battery. The first 4 digits of the designation “26650” indicate the physical dimensions while the 5th digit indicates it is a cylinder cell. The standard 26650 battery is approximately 26mm diameter by 65mm in length. This type of battery is very common in applications such as torches / flashlights, electric vehicles and various other devices that require portable power.
Some types of 26650 have been modified by adding either a button top and/or internal protection circuit. This can increase the physical length of an “26650” battery from 65mm to 72mm or in certain cases even longer. If you're using an 26650 battery for a consumer product, you should always verify with the product's manufacturer for the exact specifications required to power the device.