Historically, addiction has been defined as physical and psychological dependence on psychoactive substances (for example alcohol, tobacco, heroin, caffeine and other drugs) which cross the blood-brain barrier once ingested, temporarily altering the chemical milieu of the brain.
Addiction can also be viewed as a continued involvement with a substance or activity despite the negative consequences associated with it. Pleasure, enjoyment or relief from actual or perceived ailments would have originally been sought; however, over a period of time involvement with the substance or activity is needed to feel normal. Some psychology professionals and many laypeople now mean 'addiction' to include abnormal psychological dependency on such things as gambling, food, sex, pornography, computers, internet, work, exercise, idolizing, watching TV or certain types of non-pornographic videos, spiritual obsession, self-injury and shopping.
The American Society of Addiction Medicine begins their definition of addiction by describing it as "a primary, chronic disease of brain reward, motivation, memory and related circuitry."