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Adding search terms to a user's search. Query expansion is the process of a search engine adding search terms to a user's weighted search. The intent is to improve precision and/or recall. The additional terms may be taken from a thesaurus. For example a search for "car" may be expanded to: car cars auto autos automobile automobiles.
The additional terms may also be taken from documents that the user has specified as being relevant; this is the basis for the "more like this" feature of some search engines.
The extra terms can have positive or negative weights.