Altmetrics ("alternative metrics") let us measure and monitor the reach and impact of scholarship through online interactions. They are "alternative" to traditional measurements of academic success such as citation counts, journal prestige (impact factor), and author H-index.
Altmetrics are meant to compliment, not totally replace, traditional measures. Considering both types provides a more complete picture of how research and scholarship is used. Simply, altmetrics are metrics beyond traditional citations counts
There is increasing understanding that scholarly research has moved beyond the printed page and that traditional measures of impact offer an incomplete picture. Citations are only a small part of the scholarly ecosystem and represent only one type of impact. Other media types produced during the research process—data, tools, software, websites, videos, etc.—may be just as, or more, important than the articles that accompany them.
Since most research, including journal articles, is published electronically now, we can track how it is accessed, used, and shared. These numbers provide a more complete picture of the reach and impact of scholarship—one that goes beyond citations in peer-reviewed publications.
You probably already know that nearly everything on the internet is tracked. What you click can be used to inform website design, serve targeted adds, or as a simple measure of popularity. Altmetrics uses this ability to track interaction with online items as a way of measuring research impact and reach. Altmetrics can answer questions such as:
Confusingly there is a company named Altmetric which provides and collects altmetrics for journals and articles. Many large publishers have contracts with this company so you will see their trademark Altmetric donut (pictured here) in many places.