From the Wikipedia article for “Social“:

Social in “Socialism”

The term “socialism“, used from the 1830s onwards in France and the United Kingdom, was directly related to what was called the “social question.” In essence, early socialists contended that the emergence of competitive market societies did not create “liberty, equality and fraternity” for all citizens, requiring the intervention of politics and social reform to tackle social problems, injustices and grievances (a topic on which Jean-Jacques Rousseau discourses at length in his classic work The Social Contract). Originally the term “socialist” was often used interchangeably with “co-operative“, “mutualist“, “associationist” and “collectivist” in reference to the organization of economic enterprise socialists advocated, in contrast to the private enterprise and corporate organizational structures inherent to capitalism.

The modern concept of socialism evolved in response to the development of industrial capitalism. The “social” in modern “socialism” came to refer to the specific perspective and understanding socialists had of the development of material, economic forces and determinants of human behavior in society. Specifically, it denoted the perspective that human behavior is largely determined by a person’s immediate social environment, that modes of social organization were not supernatural or metaphysical constructs but products of the social system and social environment, which were in turn products of the level of technology/mode of production (the material world), and were therefore constantly changing. Social and economic systems were thus not the product of innate human nature, but of the underlying form of economic organization and level of technology in a given society, implying that human social relations and incentive-structures would also change as social relations and social organization changes in response to improvements in technology and evolving material forces (relations of production). This perspective formed the bulk of the foundation for Karl Marx’s materialist conception of history.