an anxiety disorder, when someone suffers from panic attacks to the point that they avoid so many situations they often become housbound. Not a fear of open spaces, in fact, when the word is broken down to original meanings, it means 'fear of the market place' in latin, which is a much more accurate description.
Derived from the Greek words Agora, meaning market and Phobia meaning fear. Agoraphobia is the fear of having a panic attack in a situation that would be hard, or embarassing to escape from. Usually accompanied and caused by GAD and/or Panic Disorder. Though cases without such have been documented, it's rarely reported due to less reason to seek medical treatment.