Product design as a verb is to create a new product to be sold by a business to its customers. A very broad concept, it is essentially the efficient and effective generation and development of ideas through a process that leads to new products.
Due to the absence of a consensually accepted definition that reflects the breadth of the topic sufficiently, two discrete, yet interdependent, definitions are needed: one that explicitly defines product design in reference to the artifact, the other that defines the product design process in relation to this artifact.
Product design as a noun: the set of properties of an artifact, consisting of the discrete properties of the form (i.e., the aesthetics of the tangible good and/or service) and the function (i.e., its capabilities) together with the holistic properties of the integrated form and function.
Product design process: the set of strategic and tactical activities, from idea generation to commercialization, used to create a product design. In a systematic approach, product designers conceptualize and evaluate ideas, turning them into tangible inventions and products. The product designer's role is to combine art, science, and technology to create new products that people can use. Their evolving role has been facilitated by digital tools that now allow designers to communicate, visualize, analyze and actually produce tangible ideas in a way that would have taken greater manpower in the past.
Product design is sometimes confused with (and certainly overlaps with) industrial design, and has recently become a broad term inclusive of service, software, and physical product design. Industrial design is concerned with bringing artistic form and usability, usually associated with craft design and ergonomics, together in order to mass-produce goods. Other aspects of product design include engineering design, particularly when matters of functionality or utility (e.g. problem-solving) are at issue, though such boundaries are not always clear.
There are various product design processes and many focus on different aspects. The process shown below, for example, is "The Seven Universal Stages of Creative Problem-Solving," outlined by Don Koberg and Jim Bagnell. It helps designers formulate their product from ideas. This process is usually completed by a group of people, i.e. industrial designers, field experts (e.g. prospective users), engineers, etc. depending upon the products involved. The process focuses on figuring out what is required, brainstorming possible ideas, creating mock prototypes, and then generating the product. However, that is not the end of the process. At this point, product designers would still need to execute the idea, making it into an actual product and then evaluate its success by seeing if any improvements are necessary.