a fundamental law of nature that is defined in various ways, such as: (a) heat will not flow spontaneously from a cold object to a hot one; (b) any system that is free of external influences becomes more disordered with time, i.e., it tends toward a state of greater entropy; (c) it is not possible to create a heat engine that extracts heat and converts all this heat to useful work, i.e., no energy-conversion process is 100% efficient. See also thermodynamics.