Article 50 isn't a particularly catchy phrase - but people are talking about it a lot right now.
So what is it and why are people discussing it?
At the moment there's a big argument about whether it should be revoked or cancelled and there's a lot of people trying to sign a petition to make that happen.
If a petition gets more than 100,000 votes, it would be considered for a debate in Parliament. Parliament's petitions website crashed on Thursday morning because of the high volume of traffic.
So what's going on? Let's go back to the beginning.
In June 2016, adults in the United Kingdom voted for the UK to leave the club of countries called the European Union.
However, not everyone was agreed on the best way of doing that.
Since then UK Prime Minister Theresa May has been trying to work out a way forward while balancing different aims; such as protecting the UK economy, opening future opportunities for the UK, sorting out issues around the border between Northern Ireland and the Republic of Ireland and keeping all the Members of Parliament (MPs) who would have to agree to vote for it happy.
She has made an agreement with the European Union leaders for a plan of how the UK would leave, but it's been rejected twice already by MPs.
They did however vote to say that they didn't want the UK to leave without a deal and that Mrs May should ask for an extension to the deadline day.
Currently, the date the UK is due to leave the European Union is 29 March 2019, but Theresa May is going to ask the European leaders for a short extension to that, to avoid the UK leaving without a deal. She says she wants that date to be no later than 30 June 2019.
But where did the date 29 March 2019 come from anyway? That's all got to do with Article 50.
Article 50 is a part of a treaty the UK signed in 2009 setting out the rules of how the European Union works.
All countries in the EU signed up to that treaty.
Article 50 of that treaty says that if a country wants to leave, the government sends a letter to the European Council - the group of EU leaders.
That triggers a two-year period during which negotiations take place to arrange a way for the country to leave that suits both sides.
It is the first time that any country has gone through this process and it's been a difficult one for Mrs May.
To get a delay to the end date - 29 March 2019 - Mrs May has to ask the European leaders to agree to it.
The European leaders may or may not agree, and may or may not put certain conditions on a delay.
If they say no, then the UK will leave as planned on 29 March but with no deal in place, unless MPs change their mind about Mrs May's deal.
Although some people would be happy with leaving without a deal, others think it would damage the UK and MPs have voted for Mrs May to avoid it.
While the UK can't decide by themselves to delay Article 50, in 2018 a European court decided that they could REVOKE - or cancel - it.
That's why some people are pushing for the prime minister to do that now as it is the sure way of making sure the UK doesn't leave at the end of March with no deal in place.
However, for many people revoking or cancelling Article 50 that would be seen as a betrayal of the referendum vote and a sign that politicians had given up on achieving Brexit.
Once the process of Article 50 has been stopped - and at the moment experts think that's very unlikely, then it could be started again but that could mean another two-year wait until a new deadline day.