Panic Disorder can be defined as the presence of recurrent, unexpected Panic Attacks (at least 2), followed by significant concern over having another attack.  A Panic Attack is marked by a discrete period of intense fear or distress in the absence of any real threat.  The attack is sudden in onset and typically peaks in ten minutes or less. This attack is accompanied by at least four of the following somatic and cognitive symptoms:
 
• Heart palpitations
• Sweating
• Trembling
• Shortness of breath or a sense of smothering
• Feelings of choking
• Chest pain
• Nausea
• Dizziness
• Fear of losing control or “going crazy”
• Fear of dying
• Paresthesias
• Chills or hot flashes
• Derealization or Depersonalization

Criteria for Agoraphobia:

• Anxiety about being in places or situations where escape may be difficult or embarrassing
• Fear that help will not be available if a panic attack occurs
• Situations are avoided or, if encountered, are endured under considerable distress

American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders, Fourth  Edition, text Revision. Washington, DC American Psychiatric Association, 2000.