Lately I have spent a lot of time defining product hypotheses and of the more contentious issues that popped up in discussions with my colleagues was the definition what constitutes a product. If we agree that a product manager is responsible for the product, common understanding of the word product is essential or we risk constant conflicts between the product manager and the rest of the stakeholders what is within the scope of project manager's responsibilities and what isn't. One way to go about defining something is to define it in a negative sense, that is, what the entity in question isn't. In the case of the word product the two neighbor concepts are a product line and a feature.
A product line is a group of related products manufactured by a single company. The defining feature of a product line is that it can be unbundled, that is, each product in the line is useful to the customer by itself and the customer is able and willing to buy it as a standalone product. On the other end of the spectrum, a feature is a distinct property or piece of something, that is, a feature is something that has little use for the customer just by itself or the customer is unable or unwilling to buy it without buying also something else. It is obvious from these definitions that delineations between a product line, a product, and a feature is very much dependent on the type of the customer and his or her needs. To avoid unproductive conflicts it is therefore advisable to avoid talking about product without talking also about its potential customers since the two cannot really be separated.