What exactly is the social and solidarity economy (SSE)? Like many terms that enter the development lexicon, its meaning is contested. Referring to the actors, institutions, principles and practices involved, this entry identifies the key features of SSE. It contrasts different perspectives on the nature and potential of SSE, highlighting both their commonalities and substantive differences. In doing so, the entry examines how SSE is positioned in the broader economy – vis-à-vis the state or public sector, the private for-profit sector and the popular or informal economy, as well as in relation to the possibilities for systemic change. It also questions whether the meaning of SSE is being diluted as the term is mainstreamed. To guard against this possibility, the entry suggests the need for an encompassing definition that acknowledges both its attributes associated with social and environmental purpose, and its democratic and transformative potential.