The h index is the number of published papers (represented by h) that have h total citations, as established by Jorge Hirsch in the journal of the Proceedings of the National Academy of Science. The index is meant as a tool to assess the output and influence of researchers.
The Web of Science, previously known as ISI Web of Knowledge, uses the h index. J.E. Hirsch, a professor of physics, proposed the index in 2005 by as a simple way to measure the breadth of a researcher's work and influence on other academics. The index is used less frequently in the arts and humanities than in the natural and social sciences, says Boston College. Some critics of the index consider it a blunt instrument because it does not value main authors differently than it does co-authors and because it can only increase and never decreases, thereby valuing the work of researchers with longer careers and more total publications.
Avoid comparing h index values between disciplines due to variations in the pattern of citation in different fields, warns the NCAR Library and Archives. An author's h index may be low even if he has only published a few extremely influential papers among many publications, though Hirsch specifically created the index to assess the contributions of the vast majority of researchers who are Nobel laureates or similarly famous for their published works already.