In psychiatry thought disorder or formal thought disorder is a term used to describe incomprehensible language, either speech or writing, that is presumed to reflect thinking. There are different types. For example, language may be difficult to understand if it switches quickly from one unrelated idea to other (flight of ideas) or if it is long-winded and very delayed at reaching its goal (circumstantiality) or if words are inappropriately strung together resulting in gibberish (word salad).
Psychiatrists consider Formal Thought Disorder as being one of two types of "thinking" or "thought" disorders. The other type being delusions. The latter involves "content" while the former involves "form". Although the term "thought disorder" can refer to either type, in common parlance it refers most often to a disorder of thought "form" also known as Formal Thought Disorder.
It is usually considered a symptom of psychotic mental illness, although it occasionally appears in other conditions. For example, pressured speech and flight of ideas may be present in mania. Clanging or echolalia may be present in Tourette syndrome. Eugen Bleuler, who named schizophrenia, held that its defining characteristic was a disorder of the thinking process. However, Formal thought disorder is not unique to schizophrenia or psychosis. So-called “organic” patients with a clouded consciousness, like that found in delirium, also have a formal thought disorder.However, there is a distinct clinical difference between the two. Schizophrenic or psychotic patients never demonstrate awareness nor concern about it because it results from a fundamental inability to use the same type of Aristotelian logic as everyone else doeswhereas so-called “organic” patients with a clouded consciousness usually do demonstrate awareness and concern about it, by complaining about being “confused” or “unable to think straight” because it results, instead, from various cognitive deficits