The European Union (EU) is a supranational political and economic union of 27 member states that are located primarily in Europe. The union has a total area of 4,233,255 km² (1,634,469 sq mi) and an estimated total population of nearly 447 million. The EU has often been described as a sui generis political entity (without precedent or comparison) combining the characteristics of both a federation and a confederation.

Containing 5.8 per cent of the world population in 2020, the EU generated a nominal gross domestic product (GDP) of around US$16.6 trillion in 2022, constituting approximately one sixth of global nominal GDP and the third-biggest global economy after the United States and China. Additionally, all EU states except Bulgaria have a very high Human Development Index according to the United Nations Development Programme. Its cornerstone, the Customs Union, paved the way to establishing an internal single market based on standardised legal framework and legislation that applies in all member states in those matters, and only those matters, where the states have agreed to act as one. EU policies aim to ensure the free movement of people, goods, services and capital within the internal market; enact legislation in justice and home affairs; and maintain common policies on trade, agriculture, fisheries and regional development. Passport controls have been abolished for travel within the Schengen Area. The eurozone is a group composed of the 20 EU member states that have fully implemented the economic and monetary union and use the euro currency. Through the Common Foreign and Security Policy, the union has developed a role in external relations and defence. It maintains permanent diplomatic missions throughout the world and represents itself at the United Nations, the World Trade Organization, the G7 and the G20. Due to its global influence, the European Union has been described by some scholars as an emerging superpower.

The union was established along with its citizenship when the Maastricht Treaty came into force in 1993, and was subsequently incorporated as an international law juridical person upon entry into force of the Treaty of Lisbon in 2009, but its beginnings may be traced to its earliest predecessors incorporated primarily by a group of founding states known as the Inner Six (Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany) at the start of modern institutionalised European integration in 1948 and onwards, namely to the Western Union (WU, 1954 renamed Western European Union, WEU), the International Authority for the Ruhr (IAR), the European Coal and Steel Community (ECSC), the European Economic Community (EEC, 1993 renamed European Community, EC) and the European Atomic Energy Community (Euratom), established, respectively, by the 1948 Treaty of Brussels, the 1948 London Six-Power Conference, the 1951 Treaty of Paris, the 1957 Treaty of Rome and the 1957 Euratom Treaty. These increasingly amalgamated bodies, later known collectively as the European Communities have grown since, along with their legal successor, the EU, both in size through accessions of further 21 states as well as in power through acquisitions of various policy areas to their remit by the virtue of the abovementioned treaties, as well as numerous other ones, such as the Modified Brussels Treaty, the Merger Treaty, the Single European Act, the Treaty of Amsterdam and the Treaty of Nice. In 2012, the EU was awarded the Nobel Peace Prize.

After the creation by six states, 22 other states joined the union in 1973–2013. The United Kingdom became the only member state to leave the EU in 2020; ten countries are aspiring or negotiating to join it.