The H-index is a metric used to measure the productivity and citation impact of a researcher or scholar. It is calculated based on the number of publications a researcher has and the number of citations those publications receive. For example, an H-index of 10 means the researcher has 10 papers that have each been cited at least 10 times. This index helps to assess the influence of a researcher's work in their field, providing a balance between quantity and quality of publications. It is commonly used in academia and can be found in databases like Google Scholar and Web of Science.