When it is self-published or independent from major publishers.
Kepler Interactive is not considered a major publisher, thus Expedition 33 is an indie game.
So, to become a non-indie game, you'd need a major publisher. Generally, a major publisher will have control over the game's development, whereas an Indie publisher either can't control it, or doesn't want to have control.
As a follow-up to that, there is some contention about whether a game needs to be both financially and creatively independent from its publisher, or if it needs to be both to be considered an indie game. For now, indie is considered, at minimum, creatively independent from the publisher.