A persistent fear of a specific object, activity, or situation (i.e., the phobic stimulus) out of proportion to the actual danger posed by the specific object or situation that results in a compelling desire to avoid it.
If it cannot be avoided, the phobic stimulus is endured with marked distress.

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 5th ed. Washington, DC. American Psychiatric Association. 2013. p. 827.