Anxiety about being in places or situations from which escape might be difficult (or embarrassing) or in which help may 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 a line; being on a bridge; and traveling in a bus, train, or automobile.
A panic attack is a discrete period of intense fear or discomfort, in which four (or more) of the following symptoms developed abruptly and reached a peak within 10 minutes:

palpitations, pounding heart, or accelerated heart rate
sweating
trembling or shaking
sensations of shortness of breath or smothering
feeling of choking
chest pain or discomfort
nausea or abdominal distress
feeling dizzy, unsteady, lightheaded, or faint
derealization (feelings of unreality) or depersonalization (being detached from oneself)
fear of losing control or going crazy
fear of dying
paresthesias (numbness or tingling sensations)
chills or hot flushes