A designation used to indicate a cartridge
that is loaded to significantly higher pressures, hence greater velocities, than the original cartridge. A "+P" designation means that the external dimensions are identical to that of the original, different from "Magnum
" which usually means a longer cartridge with greater case capacity. +P+ rated ammo are loaded to even higher pressures than +P.
Common cartridges which have a SAAMI or CIP +P designation are the .45 ACP, 9mmx19, .38 Special, and .44 Special. The .40 S&W does not have a +P standard and any .40 S&W ammo marketed as such are either overloaded rounds or just labeled by sneaky bastard marketroids.
Before trying to shoot +P rated ammunition in your firearm, first verify that it is rated for use with these higher pressure rounds. Most modern firearms can handle +P loads, but at the cost of accelerated wear. Although not very likely, the use of +P rated ammo in firearms that were not designed for it can result in a case failure which can lead to aKaBoom! or Kb!.