In everyday language 'danger' and 'dangerousness' denote risks and engender reactions of alarm and anxiety. Yet, legal use of the terms is widespread. A study ascertained that dangerousness is a criterion in the involuntary admission of mentally ill patients in the legal provisions of 30 (64 percent) of 47 jurisdictions in 43 countries. A semantic analysis of these legal definitions of dangerousness indicates that they lack precision and clarity. Furthermore, there is considerable variation among the legal meanings given to dangerousness. To avoid the confusion created by the combination of affectively loaded common usage and of imprecise legal definitions, it would be advisable either to abandon the use of dangerousness as a legal criterion or to provide clear definitions which would facilitate scientifically and ethically acceptable assessments of individuals' dangerousness. Tables, charts, and 11 references are given.