Both what Coco and X said are right, but I want to add on the back of Trainer Cards in Sword/Shield it says "Shiny Pokemon Found", so that's a total of three places they officially called shinies as Shiny Pokemon.
- In Black and White, the term was first used by an NPC in Nimbasa City
- On the official Pokemon Website
- In Pokemon Sword/Shield on the back of Trainer Cards
... So yeah, it seems to be pretty canon now. Mentioned by NPC's, on an in-game item, and on an official website.