A Boltzmann brain is a hypothesized self-aware entity which arises due to random fluctuations out of a state of chaos. The idea is named for physicist Ludwig Boltzmann (1844 - 1906), who had advanced an idea that the known universe arose as a random fluctuation, similar to process through which Boltzmann brains might arise.
The concept arises from the need to explain why we observe such a large degree of organization in the universe. The second law of thermodynamics states that the entropy in the universe will always increase. We may think of the most likely state of the universe as one of high entropy, closer to uniform and without order. So why is the observed entropy so low?
I guess I'm just a sucker for theories involving random quantum fluctuations and disembodied brains.