Maxwell's demon

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An imaginary creature who is able to sort hot molecules from cold molecules without expending energy, thus bringing about a general decrease in entropy and violating the second law of thermodynamics.

Wiktionary

1. n. A hypothetical creature created for a thought experiment. Maxwell’s demon, if it existed, would act as a sort of gate guard by sorting between higher-energy molecules and lower-energy molecules within an enclosed space. Because such a sorting process would tend to decrease entropy, it would violate the second law of thermodynamics.

WordNet 3.0

1. n. an imaginary creature that controls a small hole in a partition that divides a chamber filled with gas into two parts and allows fast molecules to move in one direction and slow molecules to move in the other direction through the hole; this would result in one part of the container becoming warmer and the other cooler, thus decreasing entropy and violating the second law of thermodynamics