Definition of Product Market Fit

I liked Marty Cagan's definition: "once you have achieved a threshold number (I advocate at least 6) of live, referenceable customers for a given vertical market"

I'd say you don't have product-market fit until you have closed, signed win-win sales and a working process to acquire more at a profit. But I'm not a huge fan of 'minimal viable products' and rapid iterations. You really ought to at least have a strong competence at actually delivering something that solves the problem for skeptical buyers before you go out "testing".