. The article is devoted to the study of lexicography, namely terminological lexicography of medicine. Medicine is the most important field, which has always influenced the lives of mankind, taking into account all the consequences of scientific and technological progress. The proof of this are the realities where the world is today in the 21st century, when human survival is threatened under the influence of global pandemics. The solution of the problems of fruitful communication in the field of medicine can be solved due to the work of lexicographers. It is necessary to follow strictly general lexicographic principles when compiling terminological dictionaries, taking into account the complexity of the terminology of medicine (polysemy, abbreviations, acronyms, eponyms, neologisms, etc.). Unfortunately, in this context the question of the history of compiling, collecting and processing data, structure, typology, use of English medical encyclopaedias remains open in the field of metalexicographic exploration, which indicates the relevance and originality of this study. The aim of the work is to identify the features of the English medical encyclopaedia as a functional unit in the system of English medical terminology using diachronic and synchronic approaches. The study was based on English medical encyclopaedias, which were selected from the catalogues of National Library of Medicine, Simon Frazer University Library, The British Library, The Internet Archive, Library of Congress, WorldCat, National Library of Ukraine named after V.I. Vernadskyi. The period of study covered VII century. – XXI c. The methodology was comprehensive: an analytical review of the literature was used to identify lacunas in linguistics; diachronic and synchronic approaches – to study the history of dictionaries compilation and the current state of medical terminography; method of quantitative analysis; method of critical analysis of dictionaries – for lexicographic analysis of English medical encyclopaedias at the levels of mega-, macro-, microstructures; descriptive and comparative methods – to describe and systematize the results. The study of the encyclopaedic lexicographic tradition in English medical terminology revealed that encyclopaedias of 259 dictionary reference books are presented in monolingual, bilingual and multilingual editions of various volumes in paper and electronic forms. Quantitative and typological characteristics vary from 227 different classical medical encyclopaedias to 32 medical encyclopaedic dictionaries. The results obtained in the course of the study allowed to reach the following conclusions: the English medical encyclopaedia combines at the structural levels both linguistic and encyclopaedic parameters of medical term registration depending on the user's requirements; the medical encyclopaedia performs the functions of codification, registration of a medical term, as well as accumulation and transfer of accumulated knowledge with the tools of general lexicography and lexicography for special purposes; English medical encyclopaedia is an integral part of English medical terminography as an open dynamic system that operates within the interaction of linguistic and under the influence of extralinguistic parameters. The prospects for further research are in the comprehensive study of all components of English medical terminography as a dynamic system that operates within and under the influence of scientific and technical process.