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 4,233,255.3 km2 (1,634,469.0 sq mi) and an estimated total population of about 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. Although the United Kingdom became the only member state to leave the EU in 2020, several other countries are aspiring or negotiating to join it.