“ [This is] the doctrine that we cannot accept the command of an authority, however exalted, as the ultimate basis of ethics. For whenever we are faced with a command by an authority, it is our responsibility to judge whether this command is moral or immoral. The authority may have power to enforce its commands, and we may be powerless to resist. But unless we are physically prevented from choosing the responsibility remains ours. It is our decision whether to obey a command, whether to accept authority. ” - Immanuel Kant

The US DSM IV diagnostic system describes the "essential features" of agoraphobia in the following way: "There is intense fear of, or discomfort in, settings from which escape is difficult or embarrassing, or in which help (e.g. to alleviate a panic attack) is not available."  It then adds the following three criteria:
1.) Anxiety about being in places or situations from which escape might be difficult (or embarrassing) or in which help might not be available in the event of having an unexpected or situationally predisposed panic attack or panic-like symptoms. Agoraphobic fears typically involve characteristic clusters of situations that include being outside the home alone, being in a crowd or standing in queues, being on a bridge, and travelling in a bus, train, or car. note: consider a diagnosis of specific phobia if the avoidance is limited to one or only a few specific situations, or social phobia if the avoidance is limited to social situations.
2.) The situations are avoided (e.g., travel is restricted), or else endured with marked distress or with anxiety about having a panic attack or panic-like symptoms, or require the presence of a companion.
3.) The anxiety or phobic avoidance is not better accounted for by another mental disorder, such as social phobia (e.g., avoidance limited to social situations because of fear of embarrassment), specific phobia (e.g., avoidance limited to one type of situation), obsessive compulsive disorder (e.g., avoidance of dirt in someone with an obsession about contamination), posttraumatic stress disorder (e.g., avoidance of stimuli associated with a severe stressor), or separation anxiety (e.g., avoidance of leaving home or relatives). 

For more on diagnosis and a wealth of other useful information and links, see the relevant section of Dr Phillip Long's fine Internet Mental Health site by clicking here.  Other useful agoraphobia-relevant resources can be reached through StressedtoZest's own collection of recommended websites - click here.  See too research articles selected by James at Connotea and other information on this site.  I frequently use Ost's Agoraphobia Scale to assess severity, to help in selecting desensitization targets, and to monitor progress. 

For self-help, many of the website links given above will be very helpful.  Also useful is the chapter on 'Help for phobias: exposure" in Edmund Bourne's anxiety and phobia book (see below).

Bourne, E. J.  "The anxiety & phobia workbook" 4th ed. Oakland, CA: New Harbinger, 2005.  [AbeBooks]  [Amazon UK]