Social sustainability in software development means creating and maintaining systems that promote pro-social values (e.g., human well-being, equity), both now and in the future. However, social sustainability lacks clear conceptual and methodological foundations, and often takes a back seat to speed and profit. This paper therefore reports a narrative review of existing definitions of social sustainability in software development and identifies key aspects of social sustainability including social equity, well-being, and community cohesion. Challenges around measuring and integrating social sustainability into practice are conceptually analyzed. The paper then proposes a comprehensive definition of social sustainability and outlines a roadmap for measuring and integrating social sustainability into software engineering processes.