Online learning as a concept and as a keyword has consistently been a focus of education research for over two decades. In this paper, we present results from a systematic literature review for the definitions of online learning because the concept of online learning, though often defined, has a range of meanings attached to it. Authors and scholars use the term to mean very distinct, if not contradictory concepts. We conducted systematic literature for over the last 30 years (1988 to 2018) to investigate the number and content of definitions of online learning. We collected 46 definitions from 37 resources and conducted a content analysis on these sets of definitions. Content analysis of the collected definitions led to an understanding of the core elements for defining online learning, the confusion surrounding the terms and the synonyms used for online learning. An evolution of the definition of the concept of online learning was also mapped to the evolution of technology in the last three decades.