Addiction is a primary, chronic disease with genetic, psychosocial, and environmental factors influencing its development and manifestations. The disease is often progressive and fatal. It is characterized by continuous or episodic impaired control over drinking or drug use, preoccupation with alcohol or drugs, use of alcohol or drugs despite adverse consequences, and distortion in thinking, most notably denial.
Simply put, Addiction means a person has no control over whether he or she uses a drug or drinks. Someone who's addicted has grown so used to the drug that he or she has to have it. Addiction can be physical, psychological, or both.