"Unpleasant or uncomfortable thoughts that cannot be consciously controlled by trying to turn the attention to other subjects. The worrying is often persistent, repetitive, and out of proportion to the topic worried about (it can even be about a triviality)."
American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. Fifth edition. Washington, DC. American Psychiatric Association. 2013. Internet. Accessed on June 21, 2017.