Choshen

"The Urim and Tumim--The Mysterious Priestly Accessory"

One of the special accessories that the High Priest wore is known as the Urim v'Tumim. It is a rather mysterious vestment and apparently had the power to determine Jewish law, and provide guidance for the leaders of Israel and the people. According to Rashi, Urim stands for light and Tumim stands for clarity.