Altmetrics, or alternative metrics, are new ways to track and measure scholarly influence and research impact in real time. They measure the attention an article (or any sort of online publication) has received online, including social media mentions, news coverage, downloads, bookmarks, blog mentions, etc. They are complementary to traditional bibliometrics such as the Journal Impact Factor and personal citation indices like the H-index.
"Altmetrics should be used in parallel with impact factors and citation counts to add a more nuanced, qualitative side to impact." (A Brief Introduction to Altmetrics for Researchers) See the Nature Jobs blogpost "Metrics with Meaning: Shaping Your Scholarly Identity" on how tracking scholarly scientific output can be beneficial to a researcher's career.
Many organizations are already actively collecting or displaying altmetrics data: popular scientific publishers (PLOS, Cell Press, Nature Publishing Group); platforms or online communities (ResearchGate, Mendeley, F1000, Scopus); and repositories (PMC, eScholarship@UMMS). There are more every day.
These are the article metrics for a Nature article published in 2013 by Jeanne Lawrence's team in the Dept of Cell and Developmental Biology where they demonstrated that they could silence the extra chromosome responsible for Down Syndrome. The Altmetric Attention Score is a measure of the attention an article has received online, including social media mentions, news coverage, and online reference manager counts.