AP meaning in Military and Defence ?

Armor-piercing ammunition (AP) is a type of projectile designed to penetrate either body armor or vehicle armor.

From the 1860s to 1950s, a major application of armor-piercing projectiles was to defeat the thick armor carried on many warships and cause damage to the lightly-armored interior. From the 1920s onwards, armor-piercing weapons were required for anti-tank missions.

AP rounds smaller than 20 mm are intended for lightly-armored targets such as body armor, bulletproof glass and light armored vehicles. In the anti-vehicle role, as tank armor improved during World War II newer designs began to use a smaller but dense penetrating body within a larger shell. These lightweight shells were fired at very high muzzle velocity and retained that speed and the associated penetrating power over longer distances. Designs using newer technologies no longer look like the classic artillery shell and have displaced it. Instead, the penetrator is a long rod of dense material like tungsten or depleted uranium (DU) that further improves the terminal ballistics. Whether these modern designs are considered to be AP rounds depends on the definition. Accordingly reference sources vary in whether they include or exclude them.