The paper describes the definitional strategy of the semantization of the medical terms, forming the conceptual sphere of “infectious diseases”. Definitions were found in the result of the linguistcal experiment aimed at identifying the peculiarities of the real functioning of the medical terms in the consciousness of ordinary native speakers that are at the initial level of professionalization. The respondents were asked to define the following terms from the conceptual sphere of "infectious diseases": botulism, brucellosis, malaria, pertussis, psittacosis. The test material gives grounds to say that the ordinary meaning of the term differs from its scientific content: the ordinary content of the term is not submitted in a logical form of a concept, but in the form of internal (mental) content, which is explicated from the inside out in the experimental situation. Interpretation analysis of the medical term meanings allowed identify the following significant components for native speakers: category of disease, epidemiology, etiology, symptoms, pathogenesis, age category and prognosis.