MEDICAL SUBJECT HEADINGS (MESH) is a comprehensive controlled vocabulary for the purpose of indexing journal articles and books in the life sciences; it serves as a thesaurus that facilitates searching. 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 is also used by ClinicalTrials.gov registry to classify which diseases are studied by trials registered in ClinicalTrials.gov.
MeSH was introduced in 1960, with the NLM's own index catalogue and the subject headings of the Quarterly Cumulative Index Medicus (1940 edition) as precursors. The yearly printed version of MeSH was discontinued in 2007 and MeSH is now available online only. It can be browsed and downloaded free of charge through PubMed . Originally in English, MeSH has been translated into numerous other languages and allows retrieval of documents from different languages.
* 1 Structure
* 1.1 Descriptor hierarchy * 1.2 Descriptions * 1.3 Qualifiers * 1.4 Supplements
The 2009 version of MeSH contains a total of 25,186 _subject headings_, also known as _descriptors_. Most of these are accompanied by a short description or definition, links to related descriptors, and a list of synonyms or very similar terms (known as _entry terms_). This additional information and the hierarchical structure (see below) make the MeSH essentially a thesaurus , rather than a plain subject headings list.
The _descriptors_ or _subject headings_ are arranged in a hierarchy. A given descriptor may appear at several locations in the hierarchical tree. The tree locations carry systematic labels known as _tree numbers_, and consequently one descriptor can carry several tree numbers. For example, the descriptor "Digestive System Neoplasms" has the tree numbers C06.301 and C04.588.274; C stands for Diseases, C06 for Digestive System Diseases and C06.301 for Digestive System Neoplasms; C04 for Neoplasms, C04.588 for Neoplasms By Site, and C04.588.274 also for Digestive System Neoplasms. The tree numbers of a given descriptor are subject to change as MeSH is updated. Every descriptor also carries a unique alphanumerical ID that will not change.
Most subject headings come with a short description or definition. See the MeSH description for diabetes type 2 as an example. The explanatory text is written by the MeSH team based on their standard sources if not otherwise stated. References are mostly encyclopaedias and standard textbooks of the subject areas. References for specific statements in the descriptions are not given, instead readers are referred to the bibliography.
In addition to the descriptor hierarchy, MeSH contains a small number of standard _qualifiers_ (also known as _subheadings_), which can be added to descriptors to narrow down the topic. For example, "Measles" is a descriptor and "epidemiology" is a qualifier; "Measles/epidemiology" describes the subheading of epidemiological articles about Measles. The "epidemiology" qualifier can be added to all other disease descriptors. Not all descriptor/qualifier combinations are allowed since some of them may be meaningless. In all there are 83 different qualifiers.
In addition to the descriptors, MeSH also contains some 139,000 _supplementary concept records_. These do not belong to the controlled vocabulary as such; instead they enlarge the thesaurus and contain links to the closest fitting descriptor to be used in a MEDLIN