A melhor definição na minha opinião é a presente no Wikipedia:

“The term co-creation refers to creativity where more than one person is involved. Some would argue that all creation is co-creation, just as Isaac Newton said that in his great work, he stood on the shoulders of giants. One thing that might be held to distinguish co-creation is that the product is something none of the creators could or would have achieved working alone.Among those who have helped popularise the term is management writer C K Prahalad, who argues value is increasingly being co-created by the firm and the customer, rather than being created entirely inside the firm.Co-creation is at the heart of the open source software movement, where users have full access to the source code and are empowered to make their own changes and improvements to it.Co-creation is becoming more evident in marketing, where companies such as Lego have successfully engaged many of their adult customers in designing new products, or Converse, which persuaded large numbers of its most passionate customers to create their own video advertisements for the product.Co-creation is also used to describe the process by which actors in improvisational theatre create scenes with each other where there is no script, but only a simple structure within which to work.”