The genre in short
Indie is short for independent, and it originally described a contractual fact rather than a music: who pressed the record and who distributed it. The UK indie chart started in 1980 and measured exactly that, distribution, not style. Things that sounded nothing like each other sat side by side on it.
It stopped working as a definition fairly quickly, but something survives: small budgets, decisions taken by very few people, the freedom to hand in a record that is slightly wrong. We use the label for what it still means, a way of working, and we apply it to people doing that now even when the sound has nothing to do with 1985.