People can argue what makes a company a startup or not initially, but I think this is the more important part, because it gives you the most useful fact about a startup: how you can tell when it stops being one.

If you're doing customer discovery, throwing MVPs out to see what sticks, and pivoting constantly, you're a startup. If you've found product-market fit, are making quality products, and are entrenching yourself into the market and eliminating competitors, you're a business.

Or, to paraphrase Eric Ries: a business is a predictable engine for repeatable revenue generation. The process of groping around in the dark trying to create such an engine, is a startup.