Entropy
Definition
A measure of disorder or energy dispersal in a system. Nature favors higher entropy: it is why rooms get messy and ice melts.
Symbol: · Unit: J/(mol K)
Technical definition
Entropy (S) is a state function quantifying the number of microscopic arrangements consistent with a system's macroscopic state. The second law states that the total entropy of an isolated system never decreases, and entropy generally rises with temperature, volume, and particle count.
Example
Ice melting into water increases entropy: the molecules go from a rigid crystal to a freely moving liquid with far more possible arrangements.
Disorder is a rough metaphor, not the definition. Entropy is really about energy dispersal among microstates, which is why a neatly stacked deck of hot cards can still have high entropy.