It's short for complex regional pain syndrome. An older term for it is reflex sympathetic dystrophy (RSD). It is a rare disorder of the nervous system, typically triggered by an injury like a broken bone or a sprain. The nervous system continues to feel pain even after the injury has healed, and the affected body part becomes swollen and extremely sensitive to touch, goes through changes in color and temperature, becomes sweatier and hairier, and becomes stiff and can atrophy. There is no cure but some people go into remission, especially if they are diagnosed early and start PT/OT right away. It's not well-known - I'd never heard of it before my diagnosis.