A lucid dream in which the dreamer realized that they are asleep while in the dream, making the dreamer conscious.
This is usually achieved through a dream sign, a reality check, or recognizing something as out of the ordinary and coming to the conclusion that it was impossible, or that it doesn't seem like real life. Although it is of course still possible to become lucid in a very realistic dream. Often just questioning things can lead to you realizing it is a dream.
There have been references to this kind of lucid dream for ages, including a reference in the christian bible and eastern religious works as well. It could be argued that as long as man has been dreaming, there have been a select few that became lucid.