My extended definition:
Emotions enrich, guide and enoble life. They are vital in helping us understand and behave in our environment. In relation to design, you might say that every design is emotional. Either intended or unintended, it will evoke an emotion. To design for emotion is the practice to design a product or service with the intention to evoke predefined emotions, be it positive or negative. The goal is to enrich the total experience people will have with that product or service.