Historically, addiction has been defined as physical
Physical dependence
Physical dependence refers to a state resulting from chronic use of a drug that has produced tolerance and where negative physical symptoms of withdrawal result from abrupt discontinuation or dosage reduction...

 and psychological dependence
Psychological dependence
In the APA Dictionary of Psychology, psychological dependence is defined as "dependence on a psychoactive substance for the reinforcement it provides." Most times psychological dependence is classified under addiction...

 on psychoactive substances (for example alcohol
Alcoholism is a broad term for problems with alcohol, and is generally used to mean compulsive and uncontrolled consumption of alcoholic beverages, usually to the detriment of the drinker's health, personal relationships, and social standing...

, tobacco
Tobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...

, heroin, caffeine and other drugs) which cross the blood-brain barrier
Blood-brain barrier
The blood–brain barrier is a separation of circulating blood and the brain extracellular fluid in the central nervous system . It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation. Endothelial cells restrict the diffusion...

 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.