Below are some research metrics that measure an author's impact:
h-index: Is based on a list of publications ranked in descending order by the Times Cited count. An index of h means that there are h papers that have each been cited at least h times.
G-index: Is an alternative to h-index, adds more weight to highly cited articles. Available through Publish or Perish. Was introduced in 2006 as "an improvement of the h-index of Hirsch to measure the global citation performance of a set of articles. If this set is ranked in decreasing order of the number of citations that they received, the g-index is the (unique) largest number such that the top g articles received (together) at least g2 citations [Egghe, L. (2006). Theory and practise of the g-index. Scientometrics, 69(1), 131-152. doi: 10.1007/s11192-006-01447].
i10-index: Is the number of publications with at least 10 citations. Used only by Google Scholar.
Author metrics depend on academic discipline, geography, (multi)authorship, age of researchers, time window, language, etc.