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A type of body armor designed to protect from from explosive weaponry such as anti-aircraft artillery ("flak" is a German contraction for Fliegerabwehrkanone, "aircraft-defense gun") or grenade fragments. Its prominent usage was by the US military during the Vietnam War, but cannot provide protection against higher velocity (rifle) bullets.
Compared to more modern soft bulletproof vests or plate carriers with rigid-looking front, flak jackets look more loose-fitting as if it they have soft quilt, having buttoned or zipper front top, collared appearance, and many of them feature pockets or pouches.
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This tag implicates jacket (learn more).