(Oracle, PLS, WAIS, Architext)
``Query-by-theme'' and ``concept searching'' identify the strongest overall themes in documents, using grammar, meaning (Thesaurus lookup) and/or pragmatic relationships between words. They discover words, phrase, or query that are conceptually related (eventually dynamically). With this feature, you can find documents that do not happen to contain any of the words you thought to search with, but are nonetheless relevant.
See also ``query expansion'' and ``relevance ranking''.