An 18650 battery is a type of lithium-ion rechargeable battery. The numbers "18650" refer to the battery's dimensions: it is 18mm in diameter and 65mm in length. 18650 batteries are commonly used in electronic devices such as laptops and flashlights, as well as in electric vehicles and other high-power applications. They are known for their high energy density, long lifespan, and relatively low self-discharge rate.
Some types of 18650 have been modified adding either a button top and/or internal protection circuit. This can increase the physical length of an “18650” battery from 65mm to 70mm or in certain cases even longer. If you're using an 18650 battery for a consumer product, you should always verify with that product's manufacturer for the exact specifications required to power the device.