Addiction refers to a chronic , relapsing disease, characterized by compulsive drug-seeking and drug use and by neurochemical and molecular changes in the brain.
Dependence on a drug such that stopping results in withdrawal.
Other /More definition:
Addiction refers to the physical dependence on a substance (e.g., alcohol, drugs, cigarette) such that withdrawal symptoms are experienced when deprived of that substance.
It is the state of physical or psychological dependence on a substance that develops when that substance is used over a period of time; a form of habitual dependence upon analgesics (e.g., narcotic drugs) characterized by psychological “highs” and a subsequent craving for steadily escalating doses
List of books: Addiction