Citaion Index

A citation index is a kind of bibliographic database, an index of citations between publications, allowing the user to easily establish which later documents cite which earlier documents. A form of citation index is first found in 12th-century Hebrew religious literature. Legal citation indexes are found in the 18th century and were made popular by citators such as Shepard's Citations (1873). In 1960, Eugene Garfield's Institute for Scientific Information (ISI) introduced the first citation index for papers published in academic journals, first the Science Citation Index (SCI), and later the Social Sciences Citation Index (SSCI) and the Arts and Humanities Citation Index (AHCI). The first automated citation indexing was done by CiteSeer in 1997. Other sources for such data include Google Scholar and Elsevier's Scopus.

Major citation indexing services

General-purpose academic citation indexes include:

• ISI (now part of Thomson Reuters) publishes the ISI citation indexes in print and compact disc. They are now generally accessed through the Web under the name Web of Science, which is in turn part of the group of databases in the Web of Knowledge.
• Elsevier publishes Scopus, available online only, which similarly combines subject searching with citation browsing and tracking in the sciences and social sciences.
• Indian Citation Index is an online citation data which covers peer reviewed journals published from India. It covers major subject areas such as scientific, technical, medical, and social sciences and includes arts and humanities. The citation database is the first of its kind in India.

Each of these offer an index of citations between publications and a mechanism to establish which documents cite which other documents. They differ widely in cost: the ISI databases and Scopus are available by subscription (generally to libraries).

In addition, CiteSeer and Google Scholar are freely available online.

Reference 

https://en.wikipedia.org/wiki/Citation_index