Citation indexes track references that authors put in the bibliographies of published papers. They provide a way to search for and analyze the literature in a way not possible through simple keyword/topical searching. It also provides data on the relative "impact" of journals, authors, and institutions, as well as assessing particular areas of research activity and publication. This field is called bibliometrics.
While a number of databases allow citation searching today, Web of Science (now a product of Clarivate Analytics, formerly part of Thomson Reuters) was the original citation index tool. It covers all disciplines, but is strongest in STEM subjects where journals are the predominant form of cited literature.
UT-Austin users can access the Web of Science citation indexes back to 1900.
Citation searching allows you to look forward in the literature from the starting point of a particular paper or group of papers. This approach complements the more familiar word-based literature searching, which looks backward in time. It allows you to find more documents on the same or similar topic without using any keywords or subject terms.
Counting one's citations is an important part of knowing your own scholarly impact.
Citation data is the raw material behind the Journal Impact Factor, a relative score of a journal's aggregate citation rate in recent years.
Sometimes you'll run across a mysterious reference, and you won't be able to determine what it's referring to. By searching that reference as a "cited reference" in Web of Science, you may find other, more complete citations that might solve the mystery. It doesn't matter how old the mystery item is -- if someone has cited it it will show up in the Citation Index.
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