Bleulerian psychiatry has considered thought disorder to be a pathognomonic symptom of schizophrenia. Evaluation of the Bleulerian perspective has been severely handicapped by the lack of any standard and widely agreed-on definition of thought disorder. Consequently, the conceptualization of thought disorder has tended to be quite diverse, and evaluation of thought disorder has tended to be quite unreliable. This report presents a set of definitions of linguistic and cognitive behaviors frequently observed in patients. These definitions derive from clinical experience, use an empirical approach, and avoid making inferences about underlying processes of thought. They attempt to define the broad range of language, thought, and communication behaviors observed in patients and are not limited to those considered characteristic of schizophrenia. The reliability of these definitions has been assessed using both tape-recorded and live interviews with patients, and it has been found to be quite good for most of the terms defined.