Why is it called a 510 cartridge?

Why is it called a 510 cartridge?

A descriptor of the threaded connection used to join parts, “510” literally refers to 10 threads at 5mm. 

This type of threaded connection quickly became the standard for most e-cigs and vape pens, making it easier to use 

various cartridges and pens together

KVAP Cartridges are all the stardard 510 Thead Connection.