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.
He wouldn't even go to the letter box because of his agoraphobia.
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.
I never knew your mother had Agoraphobia, that explains why I've never seen her around.