Coined in French in the early 1820s as a mere description of a pathological state of the bone, the term 'osteoporosis' made its way into the English medical vocabulary only in the twentieth century. Unlike other medical concepts, which have not been substantially altered by progress in medical research, the definition of osteoporosis has constantly reflected the state of knowledge on the phenomenon itself. Over its 170 years of evolution, this definition has continuously sought to maintain a difficult balance between physiological and clinical criteria. Even at present, while the 1990 Consensus Conference definition (centering mainly on physiological deterioration) merely implies the clinical aspect through the notions of fragility and fracture risk, others advocate restriction of the term to the clinical consequences of osteopenia. How will the concept of osteoporosis develop in the future? This article, which deals with the origin of the term and its successive definitions, is offered as a basis for further discussion on this subject of high topicality.