What is Article 50?
Article 50 of the Treaty on European Union sets out the process for a member state to leave the EU. The withdrawal process starts with a notification from a head of state or government to the European Council. Then a negotiation begins, with the European Commission on one side of the table and the departing member state on the other. For a deal to be done, both sides need to agree. On the EU side, that requires support from a qualified majority of the continuing members and from the European Parliament. If no agreement is reached within two years, the negotiating period may be extended with the unanimous agreement of the remaining member states. Otherwise, the member state leaves automatically at the end of the Article 50 period, unless they withdraw their notification to leave before then.
For more information about Article 50, read this explainer.