An author's impact on their field has traditionally been measured using citation counts, i.e. the number of academic publications he or she has authored and the number of times these publications are cited by other researchers. Thus, a simple way to demonstrate your impact is to create a comprehensive list of your publications and the number of times they have been cited.
Different indices have been created that calculate an author impact 'score' using data on their publications
h-Index (the most widely used)
The h-index identifies the highest number of an author's papers to have the same or higher number of citations. For instance, for an author to have an h-index of 7, he or she must have at least 7 publications with 7 or more citations.
Most researchers obtain their h-index from Web of Science, Scopus, or Google Scholar.
Limitations of the h-index:
g-index
Proposed in 2006 as an alternative to the h-index, the g-index attempts to give more weight to highly-cited papers. The g-index remains controversial and is not yet widely accepted.
i10-index
Used only in Google Scholar, this simple index, introduced in 2011, counts the number of publications with at least 10 citations.
m-index
The m-index takes into account years since first publication and is more relevant to an earlier career researcher than the h-index.
The Author Details page will display the h-index plus summary data on documents and citation trends.
Web of Science will display h-index, average citations per item, and a graph displaying publication/citation patterns:
Exploring Citations to Your Articles in Google Scholar Citations
A Google Profile includes the h-index and i10-index and and overview of citation activity over a number of years: