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. MeSH was introduced in the 1960s, 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; MeSH is now available only online. 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 origins. Structure MeSH vocabulary is divided into four types of terms. The main ones are the "headings" (also known as MeSH headings or descriptors), which describe the subject of each article (e.g., "Body Weight", "Brain Edema" or "Critical Care Nursing"). 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). MeSH contains approximately 30,000 entries (as of 2022) and is updated annually to reflect changes in medicine and medical terminology. MeSH terms are arranged in alphabetic order and in a hierarchical structure by subject categories with more specific terms arranged beneath broader terms. When we search for a MeSH term, the most specific MeSH terms are automatically included in the search. This is known as the extended search or explode of that MeSH term. This additional information and the hierarchical structure (see below) make the MeSH essentially a thesaurus, rather than a plain subject headings list. The second type of term, MeSH subheadings or qualifiers (see below), can be used with MeSH terms to more completely describe a particular aspect of a subject, such as adverse, diagnostic or genetic effects. For example, the drug therapy of asthma is displayed as asthma/drug therapy. The remaining two types of term are those that describe the type of material that the article represents (publication types), and supplementary concept records (SCR) which describes substances such as chemical products and drugs that are not included in the headings (see below as