A world-system is a socioeconomic system, under systems theory, that encompasses part or all of the globe, detailing the aggregate structural result of the sum of the interactions between polities. World-systems are usually larger than single states, but do not have to be global. The Westphalian System is the preeminent world-system operating in the contemporary world, denoting the system of sovereign states and nation-states produced by the Westphalian Treaties in 1648. Several world-systems can coexist, provided that they have little or no interaction with one another. Where such interactions becomes significant, separate world-systems merge into a new, larger world-system. Through the process of globalization, the modern world has reached the state of one dominant world-system, but in human history there have been periods where separate world-systems existed simultaneously, according to Janet Abu-Lughod. The most well-known version of the world-system approach has been developed by Immanuel Wallerstein. A world-system is a crucial element of the world-system theory, a multidisciplinary, macro-scale approach to world history and social change.

World-systems are defined by the existence of a division of labor. The modern world-system has a multi-state political structure (the interstate system) and therefore its division of labor is international division of labor. In the modern world-system, the division of labor consists of three zones according to the prevalence of profitable industries or activities: core, semiperiphery, and periphery. Countries tend to fall into one or another of these interdependent zones core countries, semi-periphery countries and the periphery countries.^[1][2] Resources are redistributed from the underdeveloped, typically raw materials-exporting, poor part of the world (the periphery) to developed, industrialized core.

World-systems, past world-systems and the modern world-system, have temporal features. Cyclical rhythms represent the short-term fluctuation of economy, while secular trends mean deeper long run tendencies, such as general economic growth or decline.^[3] The term contradiction means a general controversy in the system, usually concerning some short term vs. long term trade-offs. For example, the problem of underconsumption, wherein the drive-down of wages increases the profit for the capitalists on the short-run, but considering the long run, the decreasing of wages may have a crucially harmful effect by reducing the demand for the product. The last temporal feature is the crisis: a crisis occurs, if a constellation of circumstances brings about the end of the system.

The world-systems theory stresses that world-systems (and not nation states) should be the basic unit of social analysis.^[2][3] Thus, we should focus not on individual states, but on the relations between their groupings (core, semi-periphery, and periphery).

The most well-known version of the world-system approach has been developed by Immanuel Wallerstein, who has provided several definitions of what a world-system is, twice in 1974, first

"...a system is defined as a unit with a single division of labor and multiple cultural systems."^[4]

and second as

"…a social system, one that has boundaries, structures, member groups, rules of legitimation, and coherence."^[5]

In 1987, he elaborated his definition:

"...not the system of the world, but a system that is a world and which can be, most often has been, located in an area less than the entire globe. World-systems analysis argues that the units of social reality within which we operate, whose rules constrain us, are for the most part such world-systems [...]. ...there have been thus far only two varieties of world-systems: world-economies and world empires. A world-empire (examples, the Roman Empire, Han China) are large bureaucratic structures with a single political center and an axial division of labor, but multiple cultures. A world-economy is a large axial division of labor with multiple political centers and multiple cultures."^[3]

Thus, we can differentiate world-systems into politically unified (world-empires) and not unified (world-economies).^[2] Small, non-state units such as tribes are micro-systems.^[2]

World system refers to the entire world, whereas world-system is its fragment - the largest unit of analysis that makes sense.^[2] Wallerstein stresses the importance of hyphen in the title:

"... In English, the hyphen is essential to indicate these concepts. "World system" without a hyphen suggests that there has been only one world-system in the history of the world."^[3]

There is an ongoing debate among scholars whether we can talk about multiple world-systems. For those who support the multiple world-systems approach,^[6] there have been many world-systems throughout worlds history, some replacing others, as was the case when a multipolar world-system of the 13th-14th centuries was replaced by a series of consecutive Europe- and the West-centered world-systems.^[7] Others coexisted unknowingly with others, not linked to them directly or indirectly; in those cases the world-systems weren't worldwide (for example, prior to colonization of Americas, the Americas world-systems had no connection with the one encompassing Eurasia and Africa).^[8] From around 19th century onward, due to the process of globalization, many scholars agree that there has been only one world-system, that of capitalism.^[9][10] There are, however, dissenting voices, as some scholars do not support the contention that there is only one world-system in the modern day;^[11] Janet Abu-Lughod states that multiple world-systems did exist in past epochs.^[12]

The alternative approach insists that there was only one World System that originated in the Near East five^[13] or even ten^[14] thousand years ago, and gradually encompassed the whole world; thus, the present-day truly global World System can be regarded as its continuation.

• Scale (analytical tool)