It is appropriate for people to be anxious when they are experiencing a threat of some sort. When normal anxiety crosses the boundary to become a recognised clinical condition is now usually established by the diagnostic criteria in the International Classification of Diseases 10th edition – ICD 10.
An alternative diagnostic scheme is that provided by the Diagnostic and Statistical Manual. The fifth edition of this became operative in May 2013 (DSM-5). The earlier version DSM-IV TR is quite close to the present classification (TR standing for text revision). Both ICD-10 and DSM -IV are available on the Internet. DSM-5 is not currently available except by buying the (expensive) manual.
The conditions which you might like to consider are; Generalised Anxiety Disorder, Panic Disorder, and phobic disorders
Prof Malcolm Weller