18650 Battery: This is a specific size of rechargeable lithium-ion battery. The number "18650" comes from its dimensions: 18mm in diameter and 65mm in length. These batteries are commonly used in a variety of electronic devices, such as flashlights, laptops, power tools, and electric vehicles, due to their relatively high capacity and energy density.

3.7V: This is the nominal voltage of the 18650 lithium-ion battery. Fully charged, the voltage can reach around 4.2V, and it drops to about 3.0V when the battery is nearly depleted. The 3.7V is the typical operating voltage.

Battery Case/Holder: A case or holder is a protective structure designed to securely hold the 18650 battery in place. It usually ensures that the battery’s terminals are properly aligned for use in circuits and protects the battery from physical damage. The case also may offer short-circuit protection, especially in higher-end models, and can make it safer to handle and store the batteries.