Pierre Laplace's demon was a hypothetical construct which could calculate the position and motion of everything in the universe and then apply the laws of physics to determine everything that would occur afterwards. It exists as a sort of saint of determinism -- the idea that human thought is determined by biology and chemistry, which can be determined from physics, thus placing even free will under the demon's awareness.
Since Laplace's time, however, the mathematical discipline of chaos theory has made the demon a lot more hypothetical. We now know that even the tiniest inaccuracy in measurement at the start of an experiment can, in time, lead to vast differences between a predicted outcome and the actual thing. In other words, Laplace's demon would have to know the position and motion every particle in the entire universe to infinite precision, thus making it more complex than the universe itself.