Stop out
A stop out is the automatic closure of your positions when your margin level hits the minimum threshold, at the latest 50% in the EU and UK.
When your margin level reaches the stop out threshold, the broker closes your positions, usually starting with the biggest loser, until the level climbs back above the threshold.
For retail clients under ESMA and FCA rules, this closure must happen at the latest when your equity falls to 50% of the required margin. Some brokers close earlier.
A stop out is not a stop loss: it comes late, at the worst moment, and leaves your account badly damaged. In a violent gap it can be filled well below the threshold; negative balance protection then ensures you don't end up owing money.