Definition: Medusa was one of the Gorgon sisters whose hideous, snaky-haired head Perseus severed using a mirror given him by the gods so he wouldn't have to face her. Had he looked at her face, Medusa would have turned him to stone. Even after he had decapitated Medusa, Perseus couldn't look at her because her head retained its petrifying power.
Medusa wasn't always hideous, but had once been so beautiful she charmed the sea god Poseidon who then ravaged her in the temple of Athena. Athena was furious that her temple had been desecrated in this way and so she punished the temptress by turning her hair into snakes and making her so horrible men were turned to stone by just looking at her.
The Gorgon face with snaky hair and tongue sticking out is supposed to be apotropaic -- deflecting evil. The History Channel's program on Medusa suggested the enlarged eyes and teeth with sticking-out-tongue reflect the reality of a human head shortly after death.
The head of Medusa is affixed to the center of Athena's shield. This is referred to as the aegis.
