The best medical definition of addiction talks about the physical and the psychological aspect of it. We know that addictions have two key elements. Those are this tolerance and then this withdrawal.
Tolerance means that a person needs more and more of the substance, or needs to engage in more and more of the behavior, to get the same baseline level of high. So while they used to smoke one joint and get high, they now need to smoke three joints throughout a day, or they need to stay high all day.
The other aspect that I think is important to highlight here is this fact of withdrawal. The person experiences incredible physical and psychological distress when they don't have access to the substance or the behavior. Physically, this manifests itself. If the person is addicted to drugs and they stop taking the drugs, they'll experience incredible physical withdrawal symptoms.
It's also important to talk about the psychological impact of this, too. The psychological withdrawal symptoms. People experience incredible anxiety and stress if they don't have access to the drug or the behavior, or their ability to ingest or to take in the drug or engage in the behavior is limited.
Again, it's these two key elements of tolerance and withdrawal.