Psychosis is a core psychopathological concept with clinical, legal and ethical implications. DSM IV and ICD 10 define psychosis through the presence of certain symptoms, but fail to provide a description and analysis of the meaning of the concept. It is concluded that psychosis is a so-called primitive term, i.e. a term referring to a way of being of a-person-in-context that cannot be reduced to simpler elements and which, therefore, defies operationalization, but does not preclude acquisition of adequate clinical (inter-rater) reliability.