A term used by the United States National Library of Medicine (NLM) in its annual Medical Subject Fleadings (MeSH) publication. Each document in the biomedical literature in the library has been classified to all the subjects to which it pertains according to a hierarchical list of subjects reviewed and revised annually by NLM. The list is published in two forms, an alphabetical list of the subjects along with their reference code numbers (MeSH numbers), and a “tabular list,” called the tree structure, in which the subjects are categorized and subdivided, often to several levels, according to the hierarchical arrangement of the classification. It is this branching structure of the hierarchies which gives the tree structure its name.