Unlike many of the terms defined later, the words Hazard and RISK have the following characteristics: (a) They are words in common use. (b) In their ordinary dictionary meaning they are synonyms. (c) There has been and to some extent still is, sharp controversy as to the meaning to be assigned to them. (d) They are terms of a very fundamental character which are to be found, either singly or in compound with other words, in virtually every text on the subject. In consequence of this, it was decided to devote a more extensive and detailed narrative to these words and some of their derived expressions. Though a decade ago it was common for one author to use “hazard” where another would use “risk” and vice versa, it seems true that in the United Kingdom there is a growing consensus as to the general sense in which the terms are to be used. This has arisen, at least in part, because authoritative bodies have seen the need in writing official or otherwise influential reports to include a glossary of the meanings of the terms used in the reports though without necessarily seeking to impose these meanings on others. of special significance here has been the publication by the Royal Society of its Study Group Report on “Risk Assessment” in which it has defined both “hazard” and “risk”. It is our intention in defining these terms in this glossary to ensure that the definitions provided should not be inconsistent with the definition given in the Royal Society’s Study Group Report. However, these were phrased in general terms and we have seen it appropriate to use definitions, which are more specific to the field in question. The first term we shall define is “hazard”: .