An asterism in Cygnus, formed from 4 stars. Not one of the 88 constellations.
Three first-magnitude stars from three different constellations, high overhead through northern summer evenings. Not a constellation itself, just three bright things that happen to make a shape.
The Summer Triangle is centred near right ascension 19.4 hours, declination +33°. 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 Summer Triangle is not one of them — it is a pattern picked out of Cygnus. 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 Summer Triangle in its real position for wherever you are — switch on the asterism layer and it appears dashed across the constellations it runs through.