MEDLINE is one of the most important information sources in medicine and related life sciences. The database is available on the OVID platform and covers the international literature on biomedicine, including allied health fields, the biological and physical sciences, the humanities, and information science as they relate to medicine and health care. It contains bibliographic records and abstracts, and indexes information from approximately 5,600 journals published worldwide.

The National Library of Medicine (database producer) uses the Medical Subject Headings (MeSH) thesaurus for document-centered indexings.

You can also use the Term Finder in Advanced Search, thanks to which you can search for suitable terms for search queries in one place using several subject heading dictionaries, including the MeSH thesaurus. See this page for details.
A short video demonstrating practical use can be viewed at https://www.brainshark.com/wkovid/vu?pi=zHAzerbGjz34XJz0.