The gorgoneion is a decorative gorgon's head, face-on, which was quite common in Ancient Greek art for a few centuries. The gorgon features prominently in myth, and changed in form over time. Medusa was one of the three gorgons.
In its earliest depictions, the gorgon was a distorted, leering face, with serpents encircling but unattached. Over time, the face became more beautiful, and the serpents became the hair.
I based this small drawing off several coins (not mine, I don't have any, but I'd like one). It's a composite.