What does "18650" printed on Li-ion battery stands for?

Now when you have your new LED flashlight, you start looking for the best li-ion batteries money can buy. Searching the internet will not reveal what 18650 stand for. Everything is so confusing, and all those numbers: 18650, 16340, 18500? Thousands of batteries to buy online- the question is: will they fit my new flashlight?
Is this the right battery for me? Will this type of li-ion battery work in my flashlight at all?...

Do you know what all those numbers actually mean?

I will start with 18650 Li-ion battery. 

Why manufacturers use number 18650? No, these numbers have nothing to do with battery chemistry.

These numbers, printed on li-ion rechargeable battery cells are so easy to forget if you don't know what their meaning is. Numbers, and abbreviations printed on rechargeable li-ion batteries are not that complicated after all. I will explain it in my next post.