1. To fall back into a former mood, state, or way of life; especially, a bad or undesirable one, after coming out of it for a while.
2. To become ill again after seeming to have made a recovery.
3. A return to a former mood, state, or way of life; especially, a bad or undesirable one, after coming out of it for a while.
4. Backsliding; that is, to go back to a bad behavior; to regress, to revert.

The return of signs and symptoms of a disease after a patient has enjoyed a remission. For example, after treatment a patient with cancer of the colon went into remission with no sign or symptom of the tumor, remained in remission for four years, but then suffered a relapse and had to be treated once again for colon cancer.