Education, research, and contribution to society through innovation are the three missions of post-secondary educational institutions. There is a gap in understanding the concept of social innovation for post-second educational institutions. A clear definition would: (a) guide institutional strategic direction and supports, (b) recognize and reward academic research in social innovation, and (c) enable accurate measurement of outcomes and impact of social innovation activities. To redress the definitional imprecision, Walker and Avant’s method was used to conduct a concept analysis of social innovation. Four multi-disciplinary databases were searched to identify 1830 records. Antecedents, defining attributes, and consequences of social innovation were extracted from 272 of these articles. Defining attributes were reconstructed to develop a new definition. For post-secondary educational institutions, social innovation was defined as the intentional implementation of a transdisciplinary initiative to address a social challenge enabled through collaborative action leading to new or improved capabilities and relationships with community to generate evidence-informed solutions that are more effective, efficient, just, and sustainable. With greater clarity about the definition of social innovation, post-secondary educational institutions can create strategic plans and allocate resources to fulfil the Third Mission. With an evidence-informed definition, post-secondary educational institutions can develop a measurement framework to demonstrate outcomes and impacts of social innovation.