Dreaming is an imaginative process of the mind that occurs during REM in sleep. Forms of dream include the frightening or upsetting nightmare and erotic dreams with sexual images and nocturnal emission.
Dreams are, according to some psychologists (most famously, Sigmund Freud), rich in symbolism and offer a window into the unconscious mind. Interpretation of dreams is a regular part of psychoanalysis. It is said that one may control the course and content of dreams by a technique called lucid dreaming. However, this could distract one from the dream-matter provided by the unconscious mind.
Most mainstream academic psychologists do not believe that dreams have a coherent meaning. Carl Jung's view of dreams was more precise than this: that dreams have meanings, but their meanings are idiosyncratic, complicated, and not susceptible to more than vague, uncertain, and sometimes superficial interpretations. In particular, interpretation needs to be based on the thoughts of the individual dreamer, and not on any formula.
The art of interpreting dreams from a proto-pyschological point of view is known as oneiromancy. The usage of this now obselete word occurs at the conclusion of Sir Thomas Browne's 1658 Discourse The Garden of Cyrus:
Besides Hippocrates hath spoken so little, and the Oneirocritical Masters
have left such frigid interpretations from plants that there is little encouragement to dream of Paradise itself.
A dream is also a long-term hope, e.g. in I have a dream. It often has a positive connotation and is associated with achievement.
The term "dreamer" is used to ridicule someone who has hopes for something unlikely, or mistakenly believes something. This usage is especially associated with the term "pipe dream", which literally refers to a fantasy induced by opium.