An overwhelming and uncontrollable physical or psychological craving for a substance (such as drugs, alcohol) or activity (such as gambling, exercise). The need to have whatever it is that addicts crave prevents them from leading normal lives; all the addict's activities become directed towards satisfying the craving, to the detriment of health, family and other interpersonal relationships, and work. With some addictions, the body becomes so dependent on the substance that its withdrawal causes physical and mental illness. Addiction can be treated by psychotherapy using analysis, group therapy, and various behavior modification techniques. In some drug addictions, the prescription of less addictive drugs is used to relieve the symptoms of withdrawal.
You mention exercise as addiction. In my younger days missing a day in gym means cheating but it is not addiction of exercise, it is the aim of having a body to be appreciated by everybody.