A flak jacket or flak vest is a form of body armor. A flak jacket is designed to provide protection from case fragments ("frag") from high explosive weaponry, such as anti-aircraft artillery ("flak" is a German contraction for Fliegerabwehrkanone, "aircraft-defense gun"), grenade fragments, very small pellets used in shotguns such as the "Birdshot", and other lower-velocity projectiles. It is not designed to protect against bullets fired from most small arms such as rifles or handguns. However flak jackets are able to sustain certain gunshots, depending on the angle at which the shot was fired (an oblique angle for example), the caliber of the bullet, the speed of the projectile and the range from which the shot was fired.
The term "flak jacket" is often colloquially applied to newer body armor featuring protection against small arms projectiles, but the original usage predated the existence of modern and more resistant bulletproof vests and the two are not interchangeable in performance.