Communism (from Latin lang: communis = "common") is a socioeconomic structure and political ideology that promotes the establishment of an egalitarian, classless, stateless society based on common ownership and control of the means of production and property in general. In political science, the term "communism" is sometimes used to refer to communist states, a form of government in which the state operates under a one-party system and declares allegiance to Marxism-Leninism or a derivative thereof, even if the party does not actually claim that it has already developed communism.

Forerunners of communist ideas existed already since antiquity and then in particular in the 18th and early 19th century France, with thinkers like Jean-Jacques Rousseau and even more radical Gracchus Babeuf. The egalitarianism then emerged as a significant political power in the first half of 19th century in Western Europe. In the world shaped by the Industrial Revolution and the French Revolution, the newly established political left included many various political and intellectual movements, which are the direct ancestors of today's communism and socialism – these two then newly minted words were almost interchangeable in the time – and of anarchism or anarcho-communism. The two by far most influential theoreticians of communism of the 19th century were Germans Karl Marx and Friedrich Engels, authors of The Communist Manifesto (1848), who also helped to form the first openly communist political organizations and firmly tied communism with the idea of revolution conducted by the exploited working class. Karl Marx posited that communism would be the final stage in human society, which would be achieved after an intermediate stage called the revolutionary dictatorship of the proletariat. Communism in the Marxian sense refers to a classless, stateless and oppression-free society where decisions on what to produce and what policies to pursue are made democratically, allowing every member of society to participate in the decision-making process in both the political and economic spheres of life. Some "revisionist" Marxists of the following generations, henceforth known as socialists or social democrats, slowly drifted away from the radical views of Marx after his death in 1883; other communists, like Vladimir Lenin, continued to prepare world revolution.

The communist left, led by Vladimir Lenin, successfully came to power in Russia (1917), disrupted by the World War I. After years of civil war (1917–1921), international isolation and internal struggle in the Communist party, the Soviet Union under Joseph Stalin emerged as a new global superpower on the victorious side of the World War II. In the five years after the World War, communist regimes were established in many states of Central and Eastern Europe and in China. Communism began to spread its influence in the Third World while continuing to be a significant political force in many Western countries. International relations between Soviets and the West, led by USA, quickly worsened after the end of the war and there began the Cold war, a continuing state of conflict, tension and competition between the United States and the Soviet Union and those countries' respective allies. The "Iron curtain" between West and East then divided Europe and world from the mid-1940s to the early 1990s. Despite many communist successes like the victorious Vietnam War (1959-1975) or the first human spaceflight (1961), the communist regimes were in long term unable to keep up with the West. People under communist regimes showed their discontent in events like the Hungarian Revolution of 1956, Prague Spring of 1968 or Polish Solidarity movement in early 1980s. Since 1985, the last Soviet leader Mikhail Gorbachev tried to implement market and democratic reforms under devices like perestroika ("restructuring") and glasnost ("transparency"). His reforms sharpened internal conflicts in the communist regimes and quickly led to Revolutions of 1989, a total collapse of European communist regimes outside of Soviet Union, which dissolved itself two years later, in 1991. Some communist regimes outside of Europa survive till now, the most important of them is People's Republic of China, trying to introduce market reforms without rapid democratization.