Entropy
Definition
A measure of disorder — or more precisely, of how spread out and unusable energy has become. The universe's entropy always increases.
Symbol: · Unit: joules per kelvin (J/K)
Technical definition
A state function S quantifying the number of microscopic arrangements consistent with a system's macroscopic state. The second law of thermodynamics states that the entropy of an isolated system never decreases: ΔS ≥ 0.
Example
Drop an ice cube into warm soda: the ordered crystal melts, energy spreads evenly through the drink, and entropy rises. You will never see the drink spontaneously re-freeze the cube — the reverse would lower entropy, which the second law forbids.
Entropy is not just 'messiness.' A tidy stack of papers and a scattered pile can have similar entropy if the energy distribution is the same — entropy counts microscopic arrangements, not how neat things look. The deep statement is about energy spreading: concentrated, useful energy always degrades into diffuse, useless warmth, which is why perpetual motion machines are impossible.