A dream is the experience of envisioned images, voices, or other sensations during sleep. Dreams often portray events which are impossible or unlikely in reality, and are usually outside the control of the dreamer, (although some people have reported lucid dreaming, realizing or recognizing a reoccuring dream while they are 'in it', and then altering its reality: flying, or other supernatural feats). Many people report experiencing strong emotions while dreaming, and frightening or upsetting dreams are referred to as nightmares. The scientific discipline of dream research is oneirology.

Dreams have a long history both as a subject of conjecture and as a source of inspiration (artistic or otherwise). Throughout history, people have sought meaning in dreams. They have been described physiologically as a response to neural processes during sleep, psychologically as reflections of the unconscious, and spiritually as messages from God or predictions of the future (oneiromancy).

Most scientists believe that almost all humans dream with approximately the same frequency. Even those who rarely recall dreams report having them if awakened during rapid eye movement REM sleep. Dreaming in animals varies from species to species. Some species do not dream at all.