Medical Subject Headings (MeSH) is a comprehensive controlled vocabulary for the purpose of indexing journal articles and books in the life sciences.

MeSH is also used by ClinicalTrials.gov registry to classify which diseases are studied by trials registered in ClinicalTrials.

This additional information and the hierarchical structure (see below) make the MeSH essentially a thesaurus, rather than a plain subject headings list.

[6] The explanatory text is written by the MeSH team based on their standard sources if not otherwise stated.

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 MEDLINE search.

In MEDLINE/PubMed, every journal article is indexed with about 10–15 subject headings, subheadings and supplementary concept records, with some of them designated as major and marked with an asterisk, indicating the article's major topics.

When performing a MEDLINE search via PubMed, entry terms are automatically translated into (i.e., mapped to) the corresponding descriptors with a good degree of reliability; it is recommended to check the 'Details tab' in PubMed to see how a search formulation was translated.

PubMed does not apply automatic mapping of the term in the following circumstances: by writing the quoted phrase (e.g. "kidney allograft"), when truncated on the asterisk (e.g. kidney allograft *), and when looking with field labels (e.g. Cancer [ti]).

The XML file also has a comment that says: "the assignment of MeSH keywords is done by imperfect algorithm".