A search for information using software designed to query multiple networked information resources via a single interface. The metasearch engines developed in the second half of the 1990s were capable of searching only publicly accessible Web sites. However, the new generation of federated search engines available in the 21st century are designed to search local and remote library catalogs, abstracting and indexing databases, full-text aggregator databases, and digital repositories using standardized protocols, such as Z39.50. Some federated search systems provide deduping and rank results by relevance or allow sorting by other criteria. The current lack of a uniform authentication standard means that some databases are not accessible to federated search services. Click here to learn more about federated searching, courtesy of the Association for Library Collections and Technical Services (ALCTS) of the American Library Association (ALA), and here to learn more about the limitations of federated searching, courtesy of Information Today, Inc. See also "Federated Searching: Friend or Foe?" by William Baer in the October, 2004, issue of C&RL News.

From ODLIS, Online Dictionary for Libary and Information Science by Joan M. Reitz
http://lu.com/odlis/about.cfm