I think the most accurate definition is "produced without a publisher". That's the thing they're independent of.
Where it gets grey is things like "okay it has a publisher, but they're tiny and cute and only publish indie-feeling game"
So the "feeling" of indie is "made by a small group of people with a shared cohesive vision and minimal outside influence", where publishers tend to be large sources of outside influence, and teams larger than, like, 50 people tend to be unable to all have a truly shared cohesive vision. And a single person, by definition, always has a shared vision no matter what they make. But if a publisher is low-touch, or brings a game that's basically done to market without asking for changes, and the game feels cohesive and personal, then that's good enough.




Can confirm. Played that game, died.