A metanoia is a complete and profound transformation of one's perspective or way of life. A supervillain who decides to fight on the side of good instead of the side of evil has undergone a metanoia.
While a metanoia is a change of mind or heart, it's much deeper and more profound than, say, suddenly preferring dark chocolate over milk chocolate. The term is often used in religious contexts, referring to conversions or repentance, a turning away from a sinful life toward a new, spiritual path. But a metanoia can be any psychological or philosophical conversion, a complete reorientation of one's worldview — like swapping a high-powered, materialistic lifestyle for a quiet, simple life in a cabin in the woods.