Listen to Rabbi Abraham Twerski, who is a world expert on addictions, answer the question at this link (32 minutes and 12 seconds into the talk).
Here is a transcript:
First of all, the question is, what is the definition of addiction? I don't think we have a good definition of an addict. I think if a person knows that what he's doing is wrong, harmful, destructive, whatever, and he tries to stop doing it, and he makes a sincere effort to stop doing it, and finds that he cannot do it, I think he can call himself an addict. I don't know what the importance is for giving it a name. I think that (he can call himself an addict) if a person comes to that realization, "here's something that I know I shouldn't be doing, and I know it's destructive. I'm trying to stop it, I've tried to stop eleven times, or a hundred times, but I haven't been able to. I need someone's help at being able to stop".