Researcher impact is a measure of author productivity and impact over time. Currently, the most common measure used is the h-index which measures the impact of a particular researcher rather than a journal. The h-index is based on number of citations per publication and the total number of publications. For example, to have an h-index of 5, five of a scholar’s publications must have been cited by others at least five times each.
While more sophisticated than plain citation counts, the h-index has caveats, of course, and it's best to use this measure in context, comparing scholars with their peers, and using other metrics as well.
For a list of pros and cons of each tool, see Calculating the h-index: Web of Science, Scopus, or Google Scholar?
To calculate your h-index, list your papers based on the number of their citations, from most to least. The number of citations for each paper must be equal to or greater than its rank in order to be counted. Thus, if your first paper has at least one citation, your h index is at least one. If your second paper has at least two citations, your h-index is at least two, and so on. If you have papers A, B, C, D, and E, with 68, 12, 10, 3, and 2, respectively, your h-index is 3, because paper D (your fourth paper) must have more than four citations to be counted.
Content courtesy of Oregon State University Library's Research Metrics guide.