flak jacket

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A bulletproof jacket or vest.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A special piece of clothing worn by soldiers to protect them from shrapnel and grenade fragments, other non-direct fire and battlefield debris. Sometimes used informally to connotate any of the various forms of a bullet-proof vest.