How do you define product-market fit, and how do you know when you've achieved it?

You will know when you have PMF. If your customer acquisition keeps on growing on near exponential rate and not churning, that's a strong indication of PMF

Product-market fit is when your product solves a real problem, and customers can’t get enough of it. You know you’ve hit it when demand grows organically, retention is strong, and people are willing to pay without hesitation.