European Union Settlement Scheme
The European Union Settlement Scheme (also known as the EU Settlement Scheme or EUSS) is an immigration regime of the United Kingdom introduced by the Home Office in 2019, under the new Appendix EU of the UK's Immigration Rules, in response to the Brexit situation. EUSS processes the registration of nationals of EU and EFTA countries who were resident in the United Kingdom prior to the end of the Brexit transition/implementation period (which followed the technical departure of the UK from the European Union) at 11 pm GMT on 31 December 2020 ("IP completion day").
Source: Wikipedia — European Union Settlement Scheme (CC BY-SA 4.0)