What is entropy?

Entropy is a measure of a system's disorder or the number of microscopic configurations corresponding to its macroscopic state. In statistical mechanics, Boltzmann's formula S=kB ln(Ω) quantifies it, where Ω is the count of microstates. Physically, higher entropy means energy is more spread out or states are more numerous/uncertain. The Second Law of Thermodynamics says the entropy of an isolated system tends to increase – meaning systems evolve towards maximum disorder or equilibrium. Entropy also connects to information theory: it measures uncertainty or information content. In short, entropy tracks how uniformly energy or matter is distributed, and it never spontaneously decreases in a closed system.