Altmetrics let us measure and monitor the reach and impact of scholarship and research through online interactions. Altmetrics stands for "alternative metrics." The "alternative" part references 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, these traditional measures.
Supporters of the altmetrics movement believe that doing so will give a more complete picture of how research and scholarship is used. However, there are limits to this approach and caution should be used to not rely on any one particular measure in evaluating the importance of scholarship.
Simply, altmetrics are metrics beyond traditional citations.