What is the definition of addiction?

Defination: Drug addiction is a disease of the Brain, with associated abnormal behaviour resulting in dysfunction of Brain tissue

Complex: This is a difficult to answer in 400 characters. I would say that addiction is defined by the behavior that surrounds drug using behavior. This is different than simple tolerance and dependence. An individual can be dependent upon a drug without having an addiction. Therefore, addiction is more related to the cravings, manipulation, fantasies, motivations, etc. of a substance.