A Kage (影; Shadow) is the leader of one of the five most powerful hidden villages and are generally acknowledged as the most powerful ninja in their respective villages. They are collectively known as the Five Kage (五影, Gokage; Literally meaning Five Shadows).
The term "Kage" is not used to just refer to any leader of any village. Only the leaders of hidden villages of the Five Great Shinobi Countries are referred to as such. The Kage oversee the activities of their villages, from sending ninja teams out on their missions to making the hard decisions regarding the safety of their people. The first Kage was the Hokage (火影; Literally meaning "Fire Shadow"). Following the Hokage title, emerged the Kazekage (風影; Literally meaning "Wind Shadow") of Sunagakure, the Mizukage (水影; Literally meaning "Water Shadow") of Kirigakure, Raikage (雷影; Literally meaning "Lightning Shadow") of Kumogakure, and Tsuchikage (土影; Literally meaning "Earth Shadow") of Iwagakure.
Once a being is made Kage, normally by a predecessor, the title remains with that person for the rest of his or her life. While the previous Kage may retire, he may resume his role as leader if his successor dies. In the event that a Kage dies without naming a successor, an ideal heir could find a more suitable successor. A Daimyō has the authority to name a candidate Kage to serve as acting leader. Since the making of jinchūriki began, it became traditional for the host to be a relative of the current Kage, not only so that the host has deep loyalty bonds with the village, but also to serve as a display of the Kage's might.