Overpressure ammunition, commonly designated as +P or +P+, is small arms ammunition that has been loaded to a higher internal pressure than is standard for ammunition of its caliber (see internal ballistics), but less than the pressures generated by a proof round. This is done typically to produce rounds with a higher muzzle velocity and stopping power, such as ammunition used for defensive purposes. Because of this, +P ammunition is typically found in handgun calibers which might be used for defensive purposes.
Read more about Overpressure Ammunition: +P Vs. Magnum Cartridges, History, Standards, Commercially Available +P Cartridges, Custom and Handloaded Overpressure Cartridges, +P Ammunition Use, +P Ammunition and Velocity