From Web of Science:
The h-index is indicated by an orange horizontal line going through the Year / Total Year columns. The number of items above this line, which is "h" have at least "h" citations. For example, an h-index of 20 means there are 20 items that have 20 citations or more. This metric is useful because it discounts the disproportionate weight of highly cited papers or papers that have not yet been cited.
Calculating the h-index Value - The h-index factor is based on the depth of years of your product subscription and your selected timespan. Items that do not appear on the Results page will not be factored into the calculation. If your subscription depth is 10 years, then the h-index value is based on this depth even though a particular author may have published articles more than 10 years ago. Moreover, the calculation only includes items in your product - books and articles in non-covered journals are not included.
1. The h-index was developed by J.E. Hirsch and published in Proceedings of the National Academy of Sciences of the United States of America 102 (46): 16569-16572 November 15 2005.
The h-index is defined as follows:
A scientist has index h if h of his/her Np papers have at least h citations each, and the other (Np-h) papers have no more than h citations each.
It aims to measure the cumulative impact of a researcher’s output by looking at the amount of citations his/her work has received. Hirsch argues that the h-index is preferable to other single-number criteria, such as the total number of papers, the total number of citations and citations per paper. However, Hirsch provides a strong caveat:
Obviously a single number can never give more than a rough approximation to an individual’s multifaceted profile, and many other factors should be considered in combination in evaluating an individual. This and the fact that there can always be exceptions to rules should be kept in mind especially in life-changing decision such as the granting or denying of tenure.
The advantage of the h-index is that it combines an assessment of both quantity (number of papers) and quality (impact, or citations to these papers) (Glänzel, 2006). An academic cannot have a high h-index without publishing a substantial number of papers. However, this is not enough. These papers need to be cited by other academics in order to count for the h-index.
As such the h-index is said to be preferable over the total number of citations as it corrects for “one hit wonders”, i.e. academics who might have authored (or co-authored) one or a limited number of highly-cited papers, but have not shown a sustained and durable academic performance. It is also preferable over the number of papers as it corrects for papers that are not cited. Hence the h-index favours academics that publish a continuous stream of papers with lasting and above-average impact. (Bornmann & Daniel, forth).