Flak Jacket

Flak jackets are a form of padded protective clothing designed to provide at least partial protection from things such as blunt-force trauma, shrapnel and other projectiles. This can be seen when it protected Iruka Umino from kunai and fūma shuriken while he was searching for Naruto and the Scroll of Seals and was ambushed by Mizuki.

Background
They can also prevent lethal injuries from swords and other weapons as seen during Baki's fight with Hayate Gekkō. For the most part, they have superseded the plate armour widely used by shinobi during the Warring States Period and the First Shinobi World War. They are usually worn by chūnin and jōnin over their normal clothing in most villages, although the design varies among the villages and have also changed over time. The jackets are among the most iconic pieces of shinobi equipment. Most of them have pockets or scroll pouches, which can be used to store scrolls, weapons, and supplies. This allows a ninja to access them quickly and/or quietly with only one hand, as seen with Kakashi Hatake's case when he normally does this secretly to extract a kunai.

Trivia

 * In episode 190 of Naruto: Shippūden, multiple genin from Iwagakure are seen wearing flak jackets.
 * Several genin in Sunagakure such as Matsuri and Ittetsu have been seen wearing a variant of their village's standard flak jackets, items which are usually given to shinobi of chūnin rank or higher.
 * Flak jackets seemed to have been invented prior to the First Shinobi World War with the Gold and Silver Brothers, the Second Raikage, and the Second Tsuchikage being the earliest shinobi to be seen wearing them.
 * It may also be possible that flak jackets were invented during the Warring States Period, as a member of the Hagoromo clan was seen wearing armour that closely resembled a flak jacket.
 * In episode 316 of Naruto: Shippūden, many of the reincarnated shinobi are stated to be genin-level. Nevertheless, nearly all of them are seen wearing flak jackets.
 * At least two of the Hokage (possibly three) are seen to be wearing the flak jackets.