Medical Subject Headings (MeSH) is a huge controlled vocabulary (or metadata system) for the purpose of indexing journal articles and books in the life sciences. Created and updated by the United States National Library of Medicine (NLM), it is used by the MEDLINE/PubMed article database and by NLM's catalog of book holdings. MeSH can be browsed and downloaded free of charge on the Internet. The yearly printed version was discontinued in 2007.

In MEDLINE/PubMed, every journal article is indexed with some 10-15 headings or subheadings, with one or two of them designated as major and marked with an asterisk. When performing a MEDLINE search via PubMed, entry terms are automatically translated into the corresponding descriptors (see Details tab in PubMed). Furthermore, by default all the descriptors below the given one in the hierarchy are included in the search.