The Western world, also known as the West, primarily refers to various nations and states in Western Europe, Northern America, and Australasia; with some debate as to whether those in Eastern Europe and Latin America also constitute the West. The Western world likewise is called the Occident (from Latin occidens 'setting down, sunset, west') in contrast to the Eastern world known as the Orient (from Latin oriens 'origin, sunrise, east'). Definitions of the "Western world" vary according to context and perspectives; the West is an evolving concept made up of cultural, political, and economic synergy among diverse groups of people, and not a rigid region with fixed borders and members.

Some historians contend that a linear development of the West can be traced from Ancient Greece and Rome, while others argue that such a projection constructs a false genealogy. A geographical concept of the West started to take shape in the 4th century CE when Constantine, the first Christian Roman emperor, divided the Roman Empire between the Greek East and Latin West. The East Roman Empire, later called the Byzantine Empire, continued for a millennium, while the West Roman Empire lasted for only about a century and a half. Significant theological and ecclesiastical differences led Western Europeans to consider the Christians in the Byzantine Empire as heretics. In 1054 CE, when the church in Rome excommunicated the patriarch of Byzantium, the politico-religious division between the Western church and Eastern church culminated in the Great Schism or the East–West Schism. Even though friendly relations continued between the two parts of Christendom for some time, the crusades made the schism definitive with hostility. The West during these crusades tried to capture trade routes to the East and failed, it instead discovered the Americas. In the aftermath of the European colonization of the Americas, primarily involving Western European powers, an idea of the "Western" world, as an inheritor of Latin Christendom emerged. According to the Oxford English Dictionary, the earliest reference to the term "Western world" was from 1586, found in the writings of William Warner.

The countries that are considered constituents of the West vary according to perspective rather than their geographical location. Countries like Australia and New Zealand, located in the Eastern Hemisphere are included in modern definitions of the Western world, as these regions and others like them have been significantly influenced by the British—derived from colonization, and immigration of Europeans—factors that grounded such countries to the West. Depending on the context and the historical period in question, Russia was sometimes seen as a part of the West, and at other times juxtaposed with it, as well as endorsing anti-Western sentiment. The United States became more prominently featured in the conceptualizations of the West as it rose as a great power, amidst the development of communication–transportation technologies like the telegraph and railroads "shrinking" the distance between both the Atlantic Ocean shores.

At some times between the 18th century and the mid-20th century, prominent countries in the West such as the United States, Canada, Brazil, Argentina, Australia, and New Zealand have been envisioned by some as ethnocracies for Whites. Racism is claimed as a contributing factor to Western European colonization of the New World, which today constitutes much of the geographical Western world and is split between Global North and Global South. Starting from the late 1960s, certain parts of the Western world have become notable for their diversity due to immigration and changes in fertility rates. The idea of "the West" over the course of time has evolved from a directional concept to a socio-political concept—temporalized and rendered as a concept of the future bestowed with notions of progress and modernity.

The origins of Western civilization can be traced back to the ancient Mediterranean world. Ancient Greece and Ancient Rome are generally considered to be the birthplaces of Western civilization—Greece having heavily influenced Rome—the former due to its impact on philosophy, democracy, science, aesthetics, as well as building designs and proportions and architecture; the latter due to its influence on art, law, warfare, governance, republicanism, engineering and religion. Western Civilization is also closely associated with Christianity, the dominant religion in the West, with roots in Greco-Roman and Jewish thought. Christian ethics, drawing from the ethical and moral principles of its historical roots in Judaism, has played a pivotal role in shaping the foundational framework of Western societies. Earlier civilizations, such as the ancient Egyptians and Mesopotamians, had also significantly influenced Western civilization through their advancements in writing, law codes, and societal structures. The convergence of Greek-Roman and Judeo-Christian influences in shaping Western civilization has led certain scholars to characterize it as emerging from the legacies of Athens and Jerusalem, or Athens, Jerusalem and Rome.

In ancient Greece and Rome, individuals identified primarily as subjects of states, city-states, or empires, rather than as members of Western civilization. The distinct identification of Western civilization began to crystallize with the rise of Christianity during the Late Roman Empire. In this period, peoples in Europe started to perceive themselves as part of a unique civilization, differentiating from others like Islam, giving rise to the concept of Western civilization. By the 15th century, Renaissance intellectuals solidified this concept, associating Western civilization not only with Christianity but also with the intellectual and political achievements of the ancient Greeks and Romans.

Historians, such as Carroll Quigley in "The Evolution of Civilizations", contend that Western civilization was born around AD 500, after the fall of the Western Roman Empire, leaving a vacuum for new ideas to flourish that were impossible in Classical societies. In either view, between the fall of the Western Roman Empire and the Renaissance, the West (or those regions that would later become the heartland of the culturally "western sphere") experienced a period of decline, and then readaptation, reorientation and considerable renewed material, technological and political development. Classical culture of the ancient Western world was partly preserved during this period due to the survival of the Eastern Roman Empire and the introduction of the Catholic Church; it was also greatly expanded by the Arab importation of both the Ancient Greco-Roman and new technology through the Arabs from India and China to Europe.

Since the Renaissance, the West evolved beyond the influence of the ancient Greeks and Romans and the Islamic world, due to the successful Second Agricultural, Commercial, Scientific, and Industrial revolutions (propellers of modern banking concepts). The West rose further with the 18th century's Age of Enlightenment and through the Age of Exploration's expansion of peoples of European empires in the 18th and 19th centuries, particularly the globe-spanning colonial empires of Western Europe. Numerous times, this expansion was accompanied by Catholic missionaries, who attempted to proselytize Christianity.