Anglicisation (British English) or anglicization (American English) is a form of cultural assimilation whereby something non-English becomes assimilated into, influenced by or dominated by Englishness or Britishness. It can be socio-cultural, where a non-English person, people or place adopt(s) the English language or English customs; institutional, where institutions are modified to resemble or replaced with the institutions of England or the United Kingdom; or linguistic, where a foreign term or name is altered to become easier to say in English. It can also refer to the influence of English culture and business on other countries outside England or the United Kingdom, including media, cuisine, popular culture, technology, business practices, laws, or political systems.

Anglicisation first occurred in the British Isles, particularly to Celtic populations under the sovereignty of the King of England. Decline of the Celtic languages in England mostly occurred by 1000 AD, but continued up to the 18th century. In Scotland, the decline of Gaelic began under Malcolm III, such that by the mid-14th century, Scots was the dominant national language of Scotland.^: 139 In Wales, however, the Welsh language has continued to be spoken by a large part of the country's population, though the country still experienced anglicisation through colonisation, institutional reform and industrialisation.