A reasonable definition that I have applied in the past is "an activity that has negative consequences, but which one feels compelled to participate in anyway."
Withdrawal isn't actually required for someone to be addicted to something; they just have to want to keep doing it, even though it's obviously bad for them.
This definition still has flaws; am I "addicted" to eating unhealthy food? Etc.
Not a bad definition, the problem being that most negative consequences from drug abuse are due to the fact that they are illegal, and thus 1) way more expensive than they could be and 2) of bad quality.
Withdrawal isn't actually required for someone to be addicted to something; they just have to want to keep doing it, even though it's obviously bad for them.
This definition still has flaws; am I "addicted" to eating unhealthy food? Etc.