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 (2022年現在[update]) 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.