The h-index is the most widely used author metric. The h-index was developed by J. E. Hirsch (2005) as "an estimate of the importance, significance, and broad impact of a scientist’s cumulative research contributions.”
Simply put, the h-index is the number of papers (h) that have received at least h citations. For example, an author with 5 papers that have received at least 5 citations each has an h-index of 5. An author with 19 papers that have received at least 19 citations each has an h-index of 19.
The h-index isn't the only author metric. There are many more, including the g-index, m value, and i10.
Altmetrics is a term used to describe alternative metrics used to measure research impact. Altmetrics can paint a more complete picture of an individual's research impact. Instead of relying on citation counts, altmetrics include things like downloads, tweets, references in the popular press, etc. Both PlumX and Altmetrics include policy document citations.
PlumX Metrics is integrated into LSU Digital Commons, Mendeley, and Science Direct.