What meteorological event is a sudden warming of the polar stratosphere called?

A sudden warming of the polar stratosphere is called sudden stratospheric warming.

During this event, temperatures in the winter polar stratosphere can rise dramatically within only a few days. Large atmospheric waves generated in the troposphere propagate upward and disturb the polar vortex, the broad circulation of cold air surrounding a winter pole.

The warming occurs high above the weather layer, but its effects can later descend into the troposphere. A disrupted polar vortex may contribute to unusual cold spells or shifting weather patterns in middle and high latitudes, although a sudden stratospheric warming does not guarantee a particular surface outcome.

The event is commonly confused with an ordinary warm spell at the ground. It is defined by stratospheric temperature changes and wind reversals, not simply by pleasant winter weather at a location.