During the last several decades, the concept of social innovation has been a subject of scientific and practical discourse. As an important paradigm for innovation policies, social innovation is also an object of criticism and debate. Despite a significant proliferation of literature, the rate at which social innovation is a catalyst for coping with challenges of modern societies remains unclear. The goal of the paper is to gain a better understanding of social innovation by integrating past and present views on (...) the concept. Applying a historical overview covering the period from the 19th to the 21st century, we outline the milestones in the evolution of social innovation and distinguish seven trajectories that illustrate the commonalities in its interpretation. We consolidate the findings into a three-dimensional model that defines social innovation as an intervention that is targeted toward structural changes within a social dimension that, in terms of different functional settings (e.g., technological, business, organizational), are oriented on systemic improvements of societies. Reflecting on future avenues, we consider social innovation as an integrative part of a holistic intervention that acts across single societal dimensions and provides systemic impact for the sustainable development of societies. (shrink)