I was addicted to the kinds of activities, people, and stimulation that would end up causing misery to me.
I personally think of it as being the same as an addict with a driving compulsion to numb his/her inner pain & anxiety. My eldest sister was a perfect example - her true addiction was to the absence of anxiety, and when she wasn't drinking, she said, she felt "paralyzed by fear." She was addicted to that result, and was willing to take any number of chemical channels to get there.
In my own life I was compelled, day in and day out, to recreate the only world I had ever known, or else I suffered from severe anxiety. I medicated myself with situations that relieved my anxiety (on a certain level) because, as crappy as they were, they were comfortable and familiar.
I had to quit misery cold turkey, go through a program of recovery, learn new healthier behaviors, and always be careful of conditions that would trigger a relapse to old behaviors.
That's close enough to addiction to me, no matter what Mr. Webster has to say on the matter