Thousands of articles are published in research journals every year.

Most of them have lists of cited references — the authors’ acknowledgments of debt to prior research. The prior research may have been reported in journal articles, books, patents, or other types of material.

For example, the author of this article, "Trout Fishing on Mars," cites a journal article, a book, and a patent.
A citation index is a compilation of all the cited references from journal articles published during a particular year or group of years. In a citation index, you look up a reference to a work that you know to find journal articles that have cited it.

Cited reference searching is a fast and efficient way of finding journal articles that answer your questions or provide essential background information for your research.

For example...