What is an addiction?

What is an addiction?
The definition is: psychologically or physically habit-forming, as narcotics, to such an extent that its cessation causes severe trauma.

Let us say alcohol for instance, you don't need it all the time, you dont need it to survive you just really want it. Does that mean you have an addiction? You like to drink, you want to drink. You can still function without it, you might miss it, but you are not dependent. Does that mean you are addicited? Don't need alcohol, the cessation of alcohol would not cause me trauma, yet I cannot help but indulge to solve my problems.
What exactly constitutes an addiction or dependency?
Enlighten me.