An 18650 Battery is a lithium-ion rechargeable battery. The first 4 digits of the designation “18650” indicate the physical dimensions while the 5th digit indicates it is a cylinder cell. The standard 18650 battery is approximately 18mm diameter by 65mm in length. This type of battery is very common in applications such as electric bikes (e-Bikes), electric scooters, cordless power tools, laptop battery packs, torches / flashlights, electric vehicles, handheld vacuum and various other devices that require portable power.
Some types of 18650 have been modified by 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 the product's manufacturer for the exact specifications required to power the device.