Agoraphobia is the fear of being in a situation from which you can't escape and which could cause intense anxiety. As a result you avoid these situations.
It originally meant the fear of crowded places from the Latin word Agora meaning market place. It often begins as a panic disorder but as you increasingly begin to avoid situations or places where panics might occur then it develops into full blown agoraphobia.
Commonly these would include being far from home, away from loved ones, crowded public places, enclosed or confined places or even being at home alone. This fear can become very specific as it progresses, in some cases resulting in someone staying in their room or never going out.