Derailment (thought disorder) in the context of Disordered thought

A thought disorder (TD) is a multifaceted construct that reflects abnormalities in thinking, language, and communication. Thought disorders encompass a range of thought and language difficulties and include poverty of ideas, perverted logic (illogical or delusional thoughts), word salad, delusions, derailment, pressured speech, poverty of speech, tangentiality, verbigeration, and thought blocking. One of the first known public presentations of a thought disorder, specifically obsessive–compulsive disorder (OCD) as it is now known, was in 1691, when Bishop John Moore gave a speech before Queen Mary II, about "religious melancholy."

Two subcategories of thought disorder are content-thought disorder, and formal thought disorder. CTD has been defined as a thought disturbance characterized by multiple fragmented delusions. A formal thought disorder is a disruption of the form (or structure) of thought.Also known as disorganized thinking, FTD affects the form (rather than the content) of thought. FTD results in disorganized speech and is recognized as a key feature of schizophrenia and other psychotic disorders (including mood disorders, dementia, mania, and neurological diseases). Unlike hallucinations and delusions, it is an observable, objective sign of psychosis. FTD is a common core symptom of a psychotic disorder, and may be seen as a marker of severity and as an indicator of prognosis. It reflects a cluster of cognitive, linguistic, and affective disturbances that have generated research interest in the fields of cognitive neuroscience, neurolinguistics, and psychiatry.