We all experience dreams, and some of us even talk about them in graphic detail. So what is a dream? Throughout history, there have been a variety of definitions of dreams. The Mojing says, “A dream is a sleep that is thought to be real.” Dreams are the scenes perceived during sleep, and dreaming is first of all to sleep, otherwise what to dream about. Dream is the restless state of sleep; dream is the continuation of the mental activity of the waking state; dream is a kind of unintentional imagination; dream is the reflection of the objective reality, which is the elaboration of the ancient thinkers in China about the four layers of meaning of dream. Since the 1960s, different researchers have made various definitions of dreams according to their different understandings. 1962, Fox et al. considered that “a dream is any visual, auditory, or kinesthetic image that occurs during sleep.” Berger et al. defined dreams as “the occurrence of multiple sensory images and sensations that are bizarre or unreal to the subject.” All of these are high level generalizations of dreams, yet they seem to be incomplete. It is difficult to give a precise definition of dreams at once. Modern medicine considers that the definition of a dream must have the following two characteristics: 1. it must be a compound experience that occurs during sleep; 2. it is an image that is organized to produce perception, and this image can progress or change. This means that a dream is a hallucination that arises naturally, but is accepted as fact during sleep. This view is accepted by most scientists. In fact, dreams occur during fast-wave sleep, so they occur periodically at 90-minute intervals each night, and are a spontaneous mental image activity. If we add the characteristics of “spontaneity” and “periodicity” to Snyder’s view, the definition of dreams becomes more complete.