Research on service innovation appears in several research disciplines, with important contributions in marketing, management, and operations research. Although the concept is widely used, few research papers have explicitly defined service innovation. This dearth of research is the motivation for the present study. Through a systematic review of 1301 articles on service innovation appearing in academic journals between 1979 and 2014, this article examines research defining service innovation. The study identifies the key characteristics within 84 definitions of service innovation in different perspectives (assimilation, demarcation and synthesis) and shows how the meaning of the concept is changing. The review suggests that the large variety in definitions limits and hinders knowledge development of service innovation.