Mutual dreams.
In most dreams, we assume that the characters are creations of our mind; they have no identity of their own. But in mutual dreams, a character does exist independently.
What is a mutual dream? It is a phenomenon in which two people experience a dream together. The dream might have comparable elements (such as the same setting or activity) or they might be identical in virtually every aspect. A mutual dream is also called a "reciprocal dream" or "shared dream" -- or reve a deux by the French.
Mutual dreams imply an objective dreamscape. We generally believe that dreams occur within an individual's mind, in a fabricated dreamscape, but mutual dreams apparently happen in an actual "location" in a different world where the two dreamers can meet. In a non-mutual dream, we might encounter other evidence of this self-existing dreamscape -- thought-forms and other creations which have been left behind by other dreamers.
Mutual dreaming is an extension of a natural tendency. That tendency is to dream about someone who has stimulated or annoyed us during wakefulness. During our dreams, we generate scenarios in which to confront images of that person, in an attempt to resolve the issue. In many of those cases, the other person is similarly aroused, so he or she is likely to be dreaming about us. Mutual dreams take this tendency one step farther; instead of dreaming separately about one another, we dream together. However, when we dream about someone, the dream is usually not a mutual dream; the other character is merely a mind-creation rather than the dream-body of the person.
We can incubate a mutual dream. During wakefulness, we can talk to the person to agree on a dreamscape scenario which we would like to experience together. Then we incubate that dreamscape and an image of the other person. Some people have reported success in this technique; the achievement was verified when they met during wakefulness and discovered that their dreams had indeed occurred in an identical location and that the activities in both people's dreams were similar.