An asterism in Pegasus, formed from 5 stars. Not one of the 88 constellations.
Four stars marking a large, notably empty box. One corner belongs to Andromeda rather than Pegasus, which is a long-standing quirk of where the boundaries were drawn.
The Great Square of Pegasus is centred near right ascension 23.7 hours, declination +23°. It sits well north on the sky and is comfortably placed for northern hemisphere observers.
The International Astronomical Union recognises 88 constellations, and they divide the whole sky between them like countries on a map. The Great Square of Pegasus is not one of them — it is a pattern picked out of Pegasus. That makes it an asterism: a shape people recognise and name, with no official standing whatsoever.
The distinction matters less than it sounds. Asterisms are usually easier to find than the constellations containing them, which is exactly why they survive.
SkyRef can draw the Great Square of Pegasus in its real position for wherever you are — switch on the asterism layer and it appears dashed across the constellations it runs through.