The document discusses citation indexing. It defines citation indexing as a process that detects relationships between documents through citations. When one document cites another document, there is a conceptual relationship between the ideas in the two documents. The document outlines the history and development of citation indexing, including the first citation index created by Frank Shephard and important contributions by Eugene Garfield. It describes the major citation indexes produced by the Institute for Scientific Information (ISI), now Thomson Reuters, including the Science Citation Index, Social Sciences Citation Index, and Arts and Humanities Citation Index.