Remission is a temporary end (or significant reduction) in the signs and symptoms of an incurable disease. A disease is said to be incurable if there is always a chance that the patient will become ill again, no matter how long they have been in remission.
In cancer, the term 'in remission' is often used. Partial remission means the cancer is still detectable, but tumours or smaller, or - as in leukaemia - when there is less cancer throughout the body. Complete remission means that cancer cannot be detected using tests or scans - but because there is always the chance that cancer cells are still present, patients are said to be in remission rather than cured.