Dreams are part of our lives. In dreams, we completely abandon the rules and regulations of daily life and invite us to travel in the world of dreams. Everything in the dream confuses us or surprises us. In dreams, anything can happen: parents thousands of miles away come to us; two friends from different circles are actually talking; even relatives who have passed away appear in front of us.

The mystery of dreams is endless. Exploring the mystery of dreams is one of the fields of cutting-edge research in contemporary life sciences. What is the dream?

The Modern Chinese Dictionary explains it as: 'Dreams are imaginary activity in the brain caused by the fact that the local cerebral cortex has not completely stopped active during sleep.'

The Concise Dictionary of Biology explains it as: 'Dreams are a physiological phenomenon that occurs during sleep, characterized by complex, fragmented, and sometimes chaotic mental activities.'

The Dictionary of Modern Science and Technology explains it as: 'Dreams are a series of involuntary visual, auditory and kinesthetic images, as well as emotional and thinking activities, that occur in consciousness during sleep or sleep-like states.'

The Concise Dictionary of Psychology explains it as: 'Dreams are a passive and random imagination, a natural imagination that lacks the regulatory effect of the second information system.'

The Concise Encyclopedia Britannica explains it as: 'Dreams are imaginary activity that appears in the back of your head when you fall asleep. There are different understandings of the nature of dreams, or they believe that dreams are a reflection of reality, a source of foresight, and a spiritual feeling to eliminate diseases, or they believe that dreams are also a state of awakening, or they regard dreams as a subconscious activity...'

Although researchers devoted to dreams at all times and at home and abroad have many opinions on the definition of dreams, it is still difficult to give an accurate definition of 'dream'. We believe that dreams are an imaginary activity in the mind caused by the local brain tissue (cerebral cortex) that has not completely stopped excitatory activity during sleep. The content of a person's dream is related to the impression of life left in his consciousness when he is awake, but in the dream, the impression of life often becomes confused. Therefore, the content of dreams always presents a chaotic and illusory state, and is expressed in the form of imagination or illusion.

(Editor in charge：constructions)