European Union - Brexit.
'Article 50' deals with the procedure and timetable for a member state - for example the UK - to leave the European Union (EU).
Article 50 is part of the Lisbon Treaty of 2007.
It outlines a two-year timetable for negotiations, once a member state has notified its intention to withdraw from the EU.
The notification is often described as 'triggering' Article 50.
The UK formally triggered Article 50 on 29 March 2017, thereby originally establishing 29 March 2019 as the original timing of the UK's exit from the EU, on the assumption of two years' negotiation and ratification.
Following votes in the UK parliament, requests from the UK to the European Council, and decisions of the EU 27, potential Brexit Days currently include 31 October 2019.
Earlier dates are also possible, depending on negotiations and further votes in the UK parliament.
Any deal - between the EU and the member state leaving the EU - must be approved by a qualified majority of EU member states, and can be vetoed by the European Parliament.