Source : Free On-Line Dictionary of Computing

query expansion
     
         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.
     
        (1999-08-27)