The concept of social innovation has become pervasive among practitioners and academics, though its definition remains elusive. This paper seeks to address this by suggesting a distinction between structural social innovation, which refers to wide social change in scale and scope, targeted versions of social innovation, which can be either radical or complementary to current socio-economic institutions, and instrumental social innovation, when it is used to rebrand previous agendas in a way that is more appealing to stakeholders. These four types of social innovation are discussed referring to practical examples in the literature. We then explore ways in which the concept could be further developed by engaging with the concepts of socio-technical transitions and the foundational economy.