A lucid dream is a dream in which the sleeper is aware that he or she is dreaming. A lucid dreamer can actively participate in and manipulate 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. Lucidity usually begins in the midst of a dream when the dreamer realizes that the experience is not occurring in physical reality, but is a dream. Often this realization is triggered by the dreamer noticing some impossible or unlikely occurrence in the dream, such as flying or meeting the deceased.
Upon hearing about lucid dreaming for the first time, people often ask, "Why should I want to have lucid dreams? What are they good for?" If you consider that once you know you are dreaming, you are restricted only by your ability to imagine and conceive, not by laws of physics or society, then the answer to what lucid dreaming is good for is either extremely simple (anything!) or extraordinarily complex (everything!). It is easier to provide a sample of what some people have done with lucid dreaming than to give a definitive answer of its potential uses.