Entropy

I propose to call the magnitude S [energy unavailable for work] the entropy of the body, from the Greek word [trope], transformation....The energy of the universe is constant--the entropy of the universe tends toward a maximum.

Rudolph J.E. Clausius, paper of 1865

The concept traces to nineteenth-century advances in thermo-dynamics, the study of the relationships between heat (thus thermo-) and work or motion (thus dynamics)....

The First Law of Thermodynamics: Energy can neither be created nor destroyed; it can only change form --say from potential energy to work to heat and back again.

The Second Law: Heat can never spontaneously pass from a colder body to a warmer one.

....from Carnot's experiments, Clausius knew that work happens when energy
(in the form of heat) passes from a state of greater excitement to a state of lesser--that is from warmer to cooler. So heat can only be harnessed once to do work before it is spent--that is dissipated into a cooler body from which it can't be retrieved without adding more energy to the system.

In fact you'd have to add more energy to the system than there was heat to begin with. [Because] there is always waste in any transfer of heat....Even though energy can neither be created nor destroyed, it tends to dissipate, or pass from a more useful form ito a less useful one....

Entropy is Carnot's word for the amount of useless energy in a system--the amount of "dissipate," disordered, or constant-temperature energy, which cannot be converted into work....

Kelvin and Carnot assumed that the universe is a closed system...

And anyway, even if the universe as a whole is heading toward chaos, it's always possible to decrease entropy locally. Every time you clean up the mess in your office, you're producing order and decreasing entropy...Such order may come at the expense of disorder somewhere else, and of course useful energy is expended in the process. But as the sun isn't going to burn out anytime soon, you may as well cheer up and take what you can get.

Michael Macrone. EUREKA! 81 KEY IDEAS EXPLAINED.