Definition of "addiction"

I've found that many people do not believe that it's possible to be addicted to something such as sex.  However, to that end, let's examine the following definitions of "addiction":

"A habitual or compulsive involvement in an activity";

"Habitual psychological and physiological dependence on a substance or practice beyond one's voluntary control";

"The condition of being abnormally dependent on some habit"; and/or

"The state of being enslaved to a habit or practice or to something that is psychologically or physically habit-forming."