"Addiction is the physical and mental state of dependence on a substance or stimulus, to the point where withdrawal symptoms occur whenever the substance is not present in the body." 
Addiction is defined as a chronic, relapsing brain disease that is characterized by compulsive behavior even in the presence of harmful consequences. The term addiction can be used in many ways, by some it is used to indicate only a neurological dependence on particular chemicals such as drugs. Addiction can also be described as dependence on various behaviors. It is only fairly recently that addiction has been studied scientifically . Addiction is a very powerful motivator that will often lead people to take actions despite harmful consequences.
There are a couple of symptoms following addiction, such as the inability to limit the use of a substance or activity beyond need. The addicted user is forced to increase either the dosage of the substance or time spent engaging in the addictive activity, indicating that more is needed to provide the desired effect (known as tolerance). Failing to fulfill the desire often produces symptoms of withdrawal, such as irritability, anxiety, shakes, nausea. The symptoms can move past the physical and into the psychological. While addiction in itself is partly a psychological disease, continued use can further promote feelings of shame, guilt, anxiety or depression, just to name a few. Symptoms can vary depending on the substance or activity that is used.
Addiction is a disease. Progressive and incurable. Diagnosis is difficult and treatment limited.