Anxiety disorder and panic disorder

Wikipedia says: Panic Disorder is the medical term for a psychiatric condition characterized by recurring panic attacks in combination with significant behavioral change or at least a month of ongoing worry about the implications or concern about having other attacks.(DSM-IV)

This definition is very correct.

When the anxiety and panic attacks start to occur, you get conditioned. In your head, you link up certain places or things you do to the symptoms of the panic and anxiety attacks you had.

Suppose you are afraid to drive your car because you’ve had some panic attacks in it. Well, simply knowing that you will have to drive tomorrow might be enough to give you an anxiety attack today.
Fear of anticipation that is.

Because of this conditioning you get the behavioral changes Wikipedia’s definition talks about.