1: A reversal or return to a previous condition: After a period of  barking, Pretty Boy, Nicole's favorite spaniel, demonstrated a retrocession and stopped making the loud noise and curled up to go back to sleep.
2. A medical condition that reveals the signs of an illness, previously noted on the outside of a patient, that begin to appear internally: The large cancerous sore on Monica's forearm seemed to be getting smaller; however, the oncologist informed her that there appeared to be evidence that a  retrocession of the disease was developing on some of her other body parts.