1. An excessive fear of crowded public places, like markets, or of the necessity of leaving the sheltered protection of home, parents, friends, etc.: Agoraphobia is considered to be one of the most common abnormal panics known.
2. A mental disorder characterized by an irrational anxiety of leaving the familiar setting of one's home or venturing into the open: Agoraphobia is so pervasive that a large number of external life situations are entered into reluctantly, or are avoided, because they are often associated with hysterical attacks.

A common characteristic of agoraphobia is a history of apprehensive attacks in which the person experiences symptoms of extreme excitement, distortion of perceptions, and an overwhelming sense of imminent catastrophe, loss of control, or dread of public humiliation.

Agoraphobia can cause an individual to begin experiencing anxiety in anticipation of dreadful reactions.

3. Etymology: derived from the Greek agora which means an "assembly" or "market place"; not "open spaces", as is commonly stated by some writers. Again, agoraphobia refers to the fears of streets and crowded places, not to "open spaces".

The agoraphobia page of extended information.