This paper examines the evolution in the conceptualization of Social Innovation (SI) under the assumption of SI as a trans-disciplinary construct which comprises a diversity of discourses from different fields and actors. We performed a comprehensive and systematic literature review along six decades (1950-2014), extracting definitions of SI through a search of 2,339 documents in various languages retrieved from Web of Science, SCOPUS and Google scholar. To guide the inductive analysis of pluri-vocal discourses we assume innovation to be a learning-based process, introducing the notion of social practice linked to its intertwined institutional and socio-cultural dimensions. We applied mixed qualitative methodologies, combining content analysis based on a social constructionist/interpretivist ontology with cognitive mapping techniques. Our findings identify some core and secondary elements underpinning two complementary perspectives (transformative and instrumental) of SI as scientific construct. They also point to a number of promising avenues for research towards the advancement of a socio-technical theory of innovation.