The h-index is an impact factor based on the number and impact of a researcher’s publications. An h-index of 40 means that a researcher has published 40 papers that each have at least 40 citations.
h-index can be calculated from ISI Web of Science, Scopus, and Google Scholar - please note that the h-index of an author will be different in each of these databases, since they only calculate using their own journal content.
See the guide on Research Impact Factors for information on calculating h-index.