10mm Auto(metric designation 10x25mm) is a high powered pistol cartridge. It roughly matches the kinetic energy & stopping power of high power .357 Magnum loads while being semi-auto pistol cartridge.
It is the basis for .40 S&W, .357 Sig, 9x25mm Dillon, and some variants of .224 Boz(Failed PDW caliber).
Extreme Penetrator1 ammo chambered in this caliber is capable of piercing NIJ level 3A body armor, several inches of ballistic gel, and then ripping a hole through half a cinderblock.
It is used by the FBI Hostage Rescue Team and the Special Weapons And Tactics2 divisions in both pistols and also a rechambered MP5 but not the rest of the FBI because apparently the rest of the FBI have limp wrists.
It is often used for bear defense due to its high power. Despite only being a pistol cartridge, it is legal for hunting deer in most states due to that same hard hitting power.
Some notable firearms in this caliber are the Colt Delta Elite, the Bren Ten, the Glock 20, and the FK BRNO Field Pistol.
1These are essentially just somewhat pointy solid copper bullets that do not deform easily when passing through materials. Basically just civilian legal armor piercing ammo.
2Better known as SWAT (& SWAT teams)