The NiteCore NL185 14500 is a Li-ion battery for flashlights and other devices that ask a lot from their batteries. In terms of size this battery is similar to a AA battery or penlite. The main difference is that this 14500 uses Li-ion technology, instead of alkaline or Ni-MH. As such this 14500 battery can offer a higher output.
Not all flashlights can handle a 14500 battery so make sure you check this before you purchase it! When charging this battery you also need a charger that can charge 14500 batteries. A regular charger you use for AA batteries often won’t be able to charge 14500 batteries. So make sure you check this before you try to charge your batteries.