Just as the impact factor of a journal is one measure of the journal’s standing, prestige, or impact, the h-index (after Hirsch, who first proposed the measure1) is a measure of a scientist’s impact. Both the measures are based on the time-honoured system of citations.
If a scientist has an h-index of, say, 9, it means that he or she has published 9 or more papers, and at least 9 of them have each been cited 9 times. A high score thus means that the scientist has not only published more papers but also has been cited more often. The index can also be used to measure the impact of not just one scientist but even that of a group.
The values of the h-index are comparable within a given scientific field or discipline but not across disciplines: many more papers are published annually in physics and space science, for example, than in the social sciences or computer science.
How do you know what is your h-index? It is easy to do that if you have access to such subscription-based services as Scopus and Web of Knowledge. If not, Antony Williams shows how to calculate the h-index using freely available tools 2.
1 Hirsch J E. 2005. An index to quantify an individual’s scientific research output. Proceedings of the National Academy of Sciences (USA) 102: 16569–16572
Read more about whether the h-index is better than the impact factor.