Definition of entropy

Boltzmann turned the idea of “counting microstates” into an exact formula: $S = k_B \ln\Omega$. This section presents the definition, the Boltzmann constant and its unit, and tells the story — at times dramatic — of those who discovered it: from Clausius, who coined the word, to Boltzmann, whose probabilistic interpretation was accepted only after his death.

• Boltzmann’s formula
• Clausius, Boltzmann and the fate of the universe