In medical terminology, an addiction is a state in which the body relies on a substance for normal functioning and may occur along with physical dependence, as in drug addiction. When the drug or substance on which someone is dependent is suddenly removed, it will cause withdrawal, a characteristic set of signs and symptoms. Addiction is generally associated with increased drug tolerance. In physiological terms, addiction is not necessarily associated with substance abuse since this form of addiction can result from using medication as prescribed by a doctor. Physical dependence is different from psychological dependence (addiction). The latter is often characterized by a compulsive need for a drug for psychological reasons, while the former is characterized by need for the drug due to tolerance and the need to prevent withdrawal symptoms on discontinuing the use of a drug. Physical dependence however, commonly occurs with both addiction and therapeutic use of drugs.

However, common usage of the term addiction has spread to include psychological dependence. In this context, the term is used in drug addiction and substance abuse problems, but also refers to behaviours that are not generally recognized by the medical community as problems of addiction, such as compulsive overeating.

The term addiction is also sometimes applied to compulsions that are not substance-related, such as problem gambling and computer addiction. In these kinds of common usages, the term addiction is used to describe a recurring compulsion by an individual to engage in some specific activity, despite harmful consequences, as deemed by the user him self to his or hers individual's health, mental state or social life.

Source: http://www.wikipedia.com