The h-index is a metric used to measure the productivity and citation impact of a researcher or scholar. It is defined as the highest number h such that the researcher has published h papers, each of which has been cited at least h times. This means that an h-index of 10 indicates that the researcher has 10 papers that have each been cited at least 10 times. The h-index helps to assess the influence of a researcher's work within their field, making it easier to compare researchers with different publication records. It is commonly used in academia and can be found in databases like Google Scholar and Web of Science.