A semantic wiki is a wiki that has an underlying model of the knowledge described in its pages. Regular, or syntactic, wikis have structured text and untyped hyperlinks. Semantic wikis, on the other hand, provide the ability to capture or identify information about the data within pages, and the relationships between pages, in ways that can be queried or exported like a database through semantic queries. Semantic wikis were first proposed in the early 2000s, and began to be implemented seriously around 2005. As of 2021, well-known semantic wiki engines are Semantic MediaWiki and Wikibase. Key characteristics Formal notation The knowledge model found in a semantic wiki is typically available in a formal language, so that machines can process it into an entity-relationship model or relational database. The formal notation may be included in the pages themselves by users, as in Semantic MediaWiki, or it may be derived from the pages or the page names or the means of linking. For example, using a specific alternative page name might indicate that a specific type of link was intended. Providing information through a formal notation allows machines to calculate new facts (e.g. relations between pages) from the facts represented in the knowledge model. Semantic Web compatibility The technologies developed by the Semantic Web community provide one basis for formal reasoning about the knowledge model that is developed by importing this data. However, there is also a wide array of technologies that work on relational data. Example Imagine a semantic wiki devoted to food. The page for an apple would contain, in addition to standard text information, some machine-readable or at least machine-intuitable semantic data. The most basic kind of data would be that an apple is a kind of fruit—what's known as an inheritance relationship. The wiki would thus be able to automatically generate a list of fruits, simply by listing all pages that are tagged as being of type "fruit." Further semantic tags in the "apple" page could indicate other data about apples, including their possible colors and sizes, nutritional information and serving suggestions, and so on. If the wiki exports all this data in RDF or a similar format, it can then be queried in a similar way to a database—so that an external user or site could, for instance, request a list of all fruits that are red and can also be baked in a pie. History In