Excerpt: "Delusional disorder is one among several types of psychotic disorders, all of which involve grossly impaired reality testing. The core feature of delusional disorder is one or more nonbizarre delusions that last for at least one month. These delusions involve situations that could plausibly happen in life. Apart from the direct impact of the delusion, persons with this disorder appear normal to others and are able to function adequately in everyday life. If the person has a mood episode (such as depression) while having delusions, it must be relatively brief in order to warrant the delusional disorder diagnosis. The delusions must not be directly caused by substance use or a general medical condition in order to fit the criteria for this diagnosis."
Thurston, Nancy S., "Delusional Disorder" (1999). Faculty Publications - Grad School of Clinical Psychology. 285.