The 18th century lasted from January 1, 1701 (MDCCI) to December 31, 1800 (MDCCC). During the 18th century, elements of Enlightenment thinking culminated in the American, French, and Haitian Revolutions. During the century, slave trading and human trafficking expanded across the shores of the Atlantic, while declining in Russia, China, and Korea. Revolutions began to challenge the legitimacy of monarchical and aristocratic power structures, including the structures and beliefs that supported slavery. The Industrial Revolution began during mid-century, leading to radical changes in human society and the environment.

Western historians have occasionally defined the 18th century otherwise for the purposes of their work. For example, the "short" 18th century may be defined as 1715–1789, denoting the period of time between the death of Louis XIV of France and the start of the French Revolution, with an emphasis on directly interconnected events. To historians who expand the century to include larger historical movements, the "long" 18th century may run from the Glorious Revolution of 1688 to the Battle of Waterloo in 1815 or even later.

The period is also known as the "century of lights" or the "century of reason". In continental Europe, philosophers dreamed of a brighter age. For some, this dream turned into a reality with the French Revolution of 1789, though this was later compromised by the excesses of the Reign of Terror. At first, many monarchies of Europe embraced Enlightenment ideals, but in the wake of the French Revolution they feared loss of power and formed broad coalitions to oppose the French Republic in the French Revolutionary Wars.

The 18th century also marked the end of the Polish–Lithuanian Commonwealth as an independent state. Its semi-democratic government system was not robust enough to rival the neighboring states of the Prussia, Russia, and Austria, which partitioned the Polish–Lithuanian Commonwealth between themselves, changing the landscape of Central Europe and politics for the next hundred years.

The Ottoman Empire experienced an unprecedented period of peace and economic expansion, taking part in no European wars from 1740 to 1768. As a result, the empire was not exposed to Europe's military improvements of the Seven Years' War. The Ottoman Empire military consequently lagged behind and suffered several defeats against Russia in the second half of the century. In Southwest and Central Asia, Nader Shah led successful military campaigns and major invasions, which indirectly led to the founding of the Durrani Empire.

The European colonization of the Americas and other parts of the world intensified and associated mass migrations of people grew in size as part of the Age of Sail. European colonization intensified in present-day Indonesia, where the Dutch East India Company established increasing levels of control over the Mataram Sultanate. Mainland Southeast Asia would be embroiled in the Konbaung–Ayutthaya Wars and the Tây Sơn rebellion, while in East Asia, the century marked the High Qing era and the continual seclusion policies of the Tokugawa shogunate.

Various conflicts throughout the century, including the War of the Spanish Succession and the French and Indian War saw Great Britain triumphing over its European rivals to become the preeminent colonial power in Europe. However, Britain lost its colonies in North America after the American Revolutionary War, which went on to form the United States, initiating the decolonization of the Americas. The European colonization of Australia and New Zealand began during the late half of the century.

In the Indian subcontinent, the death of Mughal emperor Aurangzeb marked the end of medieval India and the beginning of an increasing level of European influence and control in the region, which coincided with a period of rapid Maratha expansion. After the reign of Aurangzeb, the Mughal Empire became less powerful. In 1739, Nader Shah invaded and defeated the Mughal Empire. Later, his general Ahmad Shah Abdali scored another defeat against the Mughals in the Third Battle of Panipat in 1761. By the middle of the century, the British East India Company began to conquer the eastern parts of India, a process which accelerated after their victory over the Mughal emperor, Nawab of Bengal and their French allies at the Battle of Plassey. Mughal emperor transformed into mere puppet of British. By the end of the century, Company rule in India had come to cover more regions within South Asia, the British would also expand to the south, participating in the Anglo-Mysore Wars against the Kingdom of Mysore, governed by Tipu Sultan and his father Hyder Ali.