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 U.S. National Library of Medicine (NLM), it is used by the Medline article database and by NLM's catalog of book holdings. MeSH can be browsed and downloaded free of charge on the internet; a printed version is published once a year.
The vocabulary and its supporting informatics systems were designed to be used both by indexing professionals and by medical staff with various degrees of computer experience. Using the vocabulary in support of database searches with the goal of scientific research often requires the help of specialized subject librarians. MeSH has a strong clinical bent, making non-clinical searches, such as those being done to support epidemiological studies, more difficult than the norm.
The 2005 version of MeSH contains a total of 22,568 subject headings, also known as descriptors. Most of these are accompanied by a short definition, links to related descriptors, and a list of synonyms or very similar terms (known as entry terms).
The descriptors are arranged in a hierarchy. A given descriptor may appear at several places in the hierarchy, similar to Wikipedia's category system. One branch of the hierarchy deals with geographical data.
In addition, MeSH contains a small number of standard qualifiers or subheadings, which can be added to descriptors to narrow the topic. For example, "Measles" is a descriptor and "epidemology" is a qualifier; "Measles/epidemology" describes the subheading of epidemological articles about Measles. Not all descriptor/qualifier combinations are allowed since some of them may be meaningless.
In Medline, 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 search in Medline, entry terms are automatically translated into the corresponding descriptor. Furthermore, by default all the descriptors below the given one in the hierarchy are included in the search.
In addition to the descriptors, MeSH also contains some 139,000 Supplementary Concept Records. These do not belong to the controlled vocabulary as such and are not used for indexing Medline articles; instead they serve as a thesaurus and contain links to the closest fitting descriptor, to be used in a Medline search. Many of these records describe chemical substances.
MeSH has been translated into numerous other languages and allows retrieval of documents from different languages.