Bibliographic coupling occurs when two works reference a common third work in their bibliographies. The coupling strength is higher the more citations the two bodies have in common, and this coupling is used to extrapolate how similar the subject matter of the two works is. Bibliographic coupling is invaluable in all fields of research since it helps the researcher to find related research done in the past. A closely-related notion is the "co-citation index," which refers to the number of times two works are cited together in subsequent literature.
The term "bibliographic coupling" was first introduced by MM Kessler in a paper published in 1963, and has been embraced in the work of the information scientist Eugene Garfield. Others have questioned the usefulness of the concept, pointing out that the two works may reference completely unrelated subject matter in the third.