According to the American Psychological Association:

"Addiction is a state of psychological or physical dependence (or both) on the use of alcohol or other drugs. The term is often used as an equivalent term for substance dependence and sometimes applied to behavioral disorders, such as sexual, internet, and gambling addictions."

And as most doctors and psychologists say, the sure way of telling if someone is addicted is when, for example, drinkers, say "I love drinking, but I am not an alcoholic."