For commercial products, I think the most useful interpretation is that a "viable product" is one that someone is willing to buy. That means implicitly that it has to provide value for the end user. One could also stretch the goal a little by saying that it means you can build a cashflow-positive business around the product. That implies both willingness to pay/end user value provided and an adressable market that can support operating the business. This latter definition avoids the case where a product fits just one single user and noone else.
