Discrete periods of sudden onset of intense apprehension, fearfulness or terror, often associated with feelings of impending doom.
During these attacks there are symptoms such as shortness of breath or smothering sensations, palpitations, pounding heart, or accelerated heart rate; chest pain or discomfort; choking; and fear of going crazy or losing control.

Panic attacks may be unexpected, in which the onset of the attack is not associated with a situational trigger and instead may be expected, in which the panic attack is associated with an obvious trigger, either internal or external.

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. Fifth Edition. Washington, DC. American Psychiatric Association. 2013. Page 826.