Without wanting to police the word, because you do you, "indie" comes from "independent", as in "independent of a publisher" (or, in music, where the word comes from, independent of a record label).

It doesn't have to be from a single developer, or even a small dev team, but rather from a team that is in some meaningful sense "independent".

This means, as an interesting example, that Minecraft was at one point about as indie as it is possible to be, and now is about as not-indie as it is possible to be.