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

References:
American Psychiatric Association. (1994). Diagnostic and statistical manual of mental disorders, fourth edition. Washington, DC: American Psychiatric Association.
National Institutes of Health, National Institute of Mental Health, NIH Publication No. 95-3879 (1995)