A relapse is a period of deterioration after a period of improvement. Relapse can apply to any deterioration after improvement; but it is most often used in reference to addictions or recovery after an injury or illness. For example, if an alcoholic manages to go months without having a drink and then after a hardship, begins to drink again. Or for example, if someone’s condition improves in the ICU for a few days and then he or she takes an unexpected turn for the worse again.