The story
From Cygnus + X + 1. From being the first discovered discrete astronomical X-ray source (an X-ray star) located in the constellation of Cygnus.
Etymology from Wiktionary, CC BY-SA 4.0.
Cygnus X-1
Today it means: The first generally accepted and confirmed black hole. A black hole in a high-mass X-ray binary (HMXB) with a blue supergiant companion (HD 226868), located in the constellation Cygnus. Full definition →