The article considers social legal aspects of concepts of “medical error”, “iatrogenesis”, “defect of medical care” in view of absence of their clear definition and understanding in public and professional communities. The usage of these concepts is distinguished by chronological continuity that is conditioned by influence of approaches existed at different stages. On the basis of analysis of scientific publications and normative legal base their genesis and peculiarities of definition are demonstrated. The social aspects affecting their transformation were revealed. It is established that initially traditional understanding of possible shortcomings in medical work was formed as conscientious delusion of physician because of insufficiency of existing medical knowledge about certain health problems. The results of original empirical study demonstrated stability of this understanding in professional community of practicing physicians. However, in conditions of development of new model of national health care based on application of new diagnostic and treatment tools oriented to individual approach to patient and under effect of mediatization of health sphere traditional understanding of medical errors became limited that resulted in occurrence of concept of “medical care defect”. However, its application is limited due to ambiguity of interpretation because of absence of its definition in legal field. In this regard, the article emphasizes necessity of legal substantiation of relevant definitions due to increasing of both appeals from patients and the number of civil and criminal medical cases and proposes to introduce into legal circulation term “omission under provision of medical care”.