Twin towns or sister cities are a form of legal or social agreement between towns, cities, counties, oblasts, prefectures, provinces, regions, states, and even countries in geographically and politically distinct areas to promote cultural and commercial ties. The modern concept of town twinning, conceived after the Second World War in 1947, was intended to foster friendship and understanding between different cultures and between former foes as an act of peace and reconciliation, and to encourage trade and tourism. By the 2000s, town twinning became increasingly used to form strategic international business links between member cities.
In the United Kingdom, the term "twin towns" is most commonly used; the term "sister cities" is generally used for agreements with towns and cities in the Americas. In mainland Europe, the most commonly used terms are "twin towns", "partnership towns", "partner towns", and "friendship towns". The European Commission uses the term "twinned towns" and refers to the process as "town twinning". Spain uses the term "ciudades hermanadas", which means "sister cities". Germany, Poland, and the Czech Republic use Partnerstadt (De) / Miasto Partnerskie (Pl) / Partnerské město (Cz), which translate as "Partner Town or City". France uses Ville Jumelée (Twinned Town or City), and Italy has Gemellaggio (twinning) and Comune gemellato (twinned municipality). In the Netherlands, the term is Stedenband (Cities-bond). In Greece, the word αδελφοποίηση (adelfopiisi – fraternisation) has been adopted. In Iceland, the terms vinabæir (friend towns) and vinaborgir (friend cities) are used. In the former Soviet Bloc, "twin towns" and "twin cities" are used, along with города-побратимы (Ru) (sworn brother cities).
The Americas, South Asia, and Australasia use the term "sister cities" or "twin cities". In China, the term is 友好城市 (yǒuhǎo chéngshì – friendship city). Sometimes, other government bodies enter into a twinning relationship, such as the agreement between the provinces of Hainan in China and Jeju-do in South Korea. The Douzelage is a town twinning association with one town from each of the member states of the European Union.
In recent years, the term "city diplomacy" has gained increased usage and acceptance, particularly as a strand of paradiplomacy and public diplomacy. It is formally used in the workings of the United Cities and Local Governments and the C40 Cities Climate Leadership Group and recognised by the USC Center on Public Diplomacy. A March 2014 debate in the British House of Lords acknowledged the evolution of town twinning into city diplomacy, particularly around trade and tourism, but also in culture and post-conflict reconciliation. The importance of cities developing "their own foreign economic policies on trade, foreign investment, tourism and attracting foreign talent" has also been highlighted by the World Economic Forum.