This is Article 50 of the Lisbon Treaty, the section that governs departures from the European Union.
Following the plebiscite in the UK in which the electorate voted 51.9% to 48.1% to Leave the European Union, this is the section that will be triggered when the UK government lodges its notice of departure.
Article 50 of the Lisbon Treaty.
Article 50
1. Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.