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Socialism is an economic and political philosophy encompassing diverse economic and social systems characterised by social ownership of the means of production, as opposed to private ownership. It describes the economic, political, and social theories and movements associated with the implementation of such systems. Social ownership can take various forms, including public, community, collective, cooperative, or employee. As one of the main ideologies on the political spectrum, socialism is considered the standard left wing ideology in most countries of the world. Types of socialism vary based on the role of markets and planning in resource allocation, and the structure of management in organizations.

Socialist systems divide into non-market and market forms. 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. Market socialism retains the use of monetary prices, factor markets and sometimes the profit motive. Socialist parties and ideas remain a political force with varying degrees of power and influence, heading national governments in several countries. Socialist politics have been internationalist and nationalist; organised through political parties and opposed to party politics; at times overlapping with trade unions and other times independent and critical of them, and present in 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, in the post-war period social democracy embraced 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.

The socialist political movement includes political philosophies that originated in the revolutionary movements of the mid-to-late 18th century and out of concern for the social problems that socialists 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. 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, academics have noted that some Western European countries have been governed by socialist parties or have mixed economies that are sometimes called " democratic socialist ". Following the revolutions of 1989, many of these countries moved away from socialism as a neoliberal consensus replaced the social democratic consensus in the advanced capitalist world, while many former socialist politicians and political parties embraced " Third Way " politics, remaining committed to equality and welfare, while abandoning public ownership and class-based politics. Socialism experienced a resurgence in popularity in the 2010s, most prominently in the form of democratic socialism.