https://www.nlm.nih.gov/research/umls/index.html

The purpose of the UMLS is to aid the development of systems that help health professionals and researchers retrieve and integrate electronic biomedical information from a variety of sources and to make it easy for users to link disparate information systems, including computer-based patient records, bibliographic databases, factual databases, and expert systems. There are three UMLS knowledge sources: UMLS Metathesaurus,Specialist Lexicon, and UMLS Semantic Network.