Maxwell demon

noun Physics.
  1. a hypothetical agent or device of arbitrarily small mass that is considered to admit or block selectively the passage of individual molecules from one compartment to another according to their speed, constituting a violation of the second law of thermodynamics.

Origin of Maxwell demon

First recorded in 1875–80; after J. C. Maxwell
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018