The aegis is usually said to be a shield or occasionally a breastplate. Sometimes it is associated wth Zeus, but more often with Athena. The aegis consists of the skin from the divine goat Almatheia, who raised Zeus on Crete, and the head of Medusa, which was a gift from Perseus to Athena.
Oxford Concise Companion to Classical Literature. ed. Howatson and Chilvers. Oxford. New York, 1993.