Socialism is a political philosophy and movement encompassing a wide range of economic and social systems, which are characterised by social ownership of the means of production, as opposed to private ownership. As a term, it describes the economic, political, and social theories and movements associated with the implementation of such systems. Social ownership can be public, community, collective, cooperative, or employee. While no single definition encapsulates the many types of socialism, social ownership is the one common element, and is considered left-wing. Different types of socialism vary based on the role of markets and planning in resource allocation, on the structure of management in organizations, and from below or from above approaches, with some socialists favouring a party, state, or technocratic-driven approach. Socialists disagree on whether government, particularly existing government, is the correct vehicle for change.

Socialist systems are divided into non-market and market forms. Non-market socialism substitutes factor markets and often money with integrated economic planning and engineering or technical criteria based on calculation performed in-kind, thereby producing a different economic mechanism that functions according to different economic laws and dynamics than those of capitalism. A non-market socialist system seeks to eliminate the perceived inefficiencies, irrationalities, unpredictability, and crises that socialists traditionally associate with capital accumulation and the profit system in capitalism. By contrast, market socialism retains the use of monetary prices, factor markets and in some cases the profit motive, with respect to the operation of socially owned enterprises and the allocation of capital goods between them. Profits generated by these firms would be controlled directly by the workforce of each firm or accrue to society at large in the form of a social dividend. Anarchism and libertarian socialism oppose the use of the state as a means to establish socialism, favouring decentralisation above all, whether to establish non-market socialism or market socialism.

Socialist politics has been both internationalist and nationalist; organised through political parties and opposed to party politics; at times overlapping with trade unions and at other times independent and critical of them, and present in both industrialised and developing nations. Social democracy originated within the socialist movement, supporting economic and social interventions to promote social justice. While retaining socialism as a long-term goal, since the post-war period it came to embrace a mixed economy based on Keynesianism within a predominantly developed capitalist market economy and liberal democratic polity that expands state intervention to include income redistribution, regulation, and a welfare state. Economic democracy proposes a sort of market socialism, with more democratic control of companies, currencies, investments, and natural resources.

The socialist political movement includes a set of political philosophies that originated in the revolutionary movements of the mid-to-late 18th century and out of concern for the social problems that were associated with capitalism. By the late 19th century, after the work of Karl Marx and his collaborator Friedrich Engels, socialism had come to signify anti-capitalism and advocacy for a post-capitalist system based on some form of social ownership of the means of production. By the early 1920s, communism and social democracy had become the two dominant political tendencies within the international socialist movement, with socialism itself becoming the most influential secular movement of the 20th century. Socialist parties and ideas remain a political force with varying degrees of power and influence on all continents, heading national governments in many countries around the world. Many socialists also adopted the causes of other social movements, such as feminism, environmentalism, and progressivism.

While the emergence of the Soviet Union as the world's first nominally socialist state led to socialism's widespread association with the Soviet economic model, several scholars posit that in practice, the model functioned as a form of state capitalism. Several academics, political commentators, and scholars have distinguished between authoritarian socialist and democratic socialist states, with the first representing the Eastern Bloc and the latter representing Western Bloc countries which have been democratically governed by socialist parties such as Britain, France, Sweden, and Western countries in general, among others. Following the end of the Cold War, many of these countries have moved away from socialism as a neoliberal consensus replaced the social democratic consensus in the advanced capitalist world.