"Online learning"--learning that involves interactions that are mediated through using digital, typically internet-based, technology--is pervasive, multi-faceted, and evolving, creating opportunities and challenges for educational research in the wake of the COVID-19 pandemic. In this special issue, we advance an interdisciplinary agenda for online learning research at the intersection of educational technology, educational psychology, and the learning sciences. In this first article in the issue, we define online learning including its variants, review historical and current contexts, explore its multidimensionality, and assert key issues shaping contemporary online learning. We propose five important lenses: "community," "engagement," "pedagogy," "equity," and "design-based research," through which scholarly communities are creating knowledge that influences research and practice in online learning contexts. These lenses highlight points of conceptual overlap between the online and psychological learning literatures. The special issue is organized around these lenses and includes an article representing each one, complemented by commentaries that offer critical appraisal and synthesis.