A lucid dream is a dream in which the sleeper is aware that he or she is dreaming. When the dreamer is lucid, he or she can actively participate in and often manipulate the imaginary experiences in the dream environment. Lucid dreams can seem extremely real and vivid depending on a person's level of self-awareness during the lucid dream.

A lucid dream can begin in one of three ways. A dream-initiated lucid dream (DILD) starts as a normal dream, and the dreamer eventually concludes that he or she is dreaming, while a wake-initiated lucid dream (WILD) occurs when the dreamer goes from a normal waking state directly into a dream state with no apparent lapse in consciousness. A mnemonic-initiated lucid dream (MILD) can happen when the dreamer intentionally affirms to himself or herself that he or she will become lucid during the upcoming sleep. This can sometimes happen due to dream-signs or spontaneously upon remembrance. Lucid dreaming has been researched scientifically, and its existence is well established. Scientists such as Allan Hobson, with his neurophysiological approach to dream research, have helped to push the understanding of lucid dreaming into a less speculative realm.