Definitions
1.nounStrictly thermodynamic entropy. A measure of the amount of energy in a physical system that cannot be used to do work.
2.nounA measure of the disorder present in a system."Ludwig Boltzmann defined entropy as being directly proportional to the natural logarithm of the number of microstates yielding an equivalent thermodynamic macrostate (with the eponymous constant of proportionality). Assuming (by the fundamental postulate of statistical mechanics), that all microstates are equally probable, this means, on the one hand, that macrostates with higher entropy are more probable, and on the other hand, that for such macrostates, the quantity of information required to describe a particular one of its microstates will be higher. That is, the Shannon entropy of a macrostate would be directly proportional to the logarithm of the number of equivalent microstates (making it up). In other words, thermodynamic and informational entropies are rather compatible, which shouldn't be surprising since Claude Shannon derived the notation 'H' for information entropy from Boltzmann's H-theorem."
3.nounThe capacity factor for thermal energy that is hidden with respect to temperature http//arxiv.org/pdf/physics/0004055.
4.nounThe dispersal of energy; how much energy is spread out in a process, or how widely spread out it becomes, at a specific temperature. https//web.archive.org/web/20060702234316/http://www.entropysite.com/students_approach.html
5.nounA measure of the amount of information and noise present in a signal.
6.nounThe tendency of a system that is left to itself to descend into chaos.