Altmetrics measure the "“the volume and nature of attention that research receives online.” (What are altmetrics?)
"Altmetrics is the study and use of non-traditional scholarly impact measures that are based on activity in web-based environments. As scholarship increasingly moves online, these metrics track associated interactions and activity to generate fine-grained data, allowing researchers and policy makers to create a higher resolution picture of the reach and impact of academic research." (PLOS Collections AltmetricsAlt)
Altmetric.com (not to be confused with altmetrics themselves) is a commercial provider of altmetrics data. Known for their "doughnut" with a score, Altmetric.com widgets appear on publisher websites and within Syracuse University Libraries' Summon search results. and even in library catalogs. Some Altmetric.com data is available for free while some requires an additional subscription.