A Shindan is a fragment of your heart when fired from a gun. A Shindan can only be formed when a golden bullet (Cortex gave Naruto, Sakura, Sai and Kakashi some golden bullets) is in the gun.
Shindan is from the anime 'Tegami Bachi', or 'Letter Bee'. Shindans can come in various types. The healing and attacking types of Shindan. The gun that is equipped with a golden bullet is called a Shindanjuu, a gun that is able to shoot out a fragment of your heart. Shindan is also a type of bullet that can destroy Gaichuus, a metallic bug that can never be destroyed, only by Shindans. In the anime 'Tegami Bachi', the golden bullet is actually called a 'Spirit Amber'. Spirit Ambers are needed to connect to the heart and they can come in diffirent colors. Some are black, blue (malice), red and yellow. A gun is needed as it helps form the heart to small bullet-like fragments and shoot out to a Gaichuu. Some Spirit Amber moves are Akabari (meaning 'Red Needle'), Kurobari (meaning 'Black Needle'), Yellow Explosion and Aotoge (meaning 'Blue Thorn').