What is entropy in plain terms?

Entropy is a measure of how many different arrangements of a system's parts would look the same from the outside. High entropy means many arrangements produce that appearance; low entropy means very few do.

Why "disorder" is a misleading shorthand. It is the usual one-word gloss and it causes confusion, because disorder is subjective and entropy is not. The precise version is about counting microstates — specific arrangements of particles — consistent with a given macrostate, the overall observable condition.

A concrete illustration. Shuffle a deck of cards. There is exactly one arrangement in perfect suit and rank order, and an astronomically large number that look shuffled. Shuffling moves you to a random arrangement, and it will essentially certainly be one of the enormously more numerous disordered ones — not because order is forbidden, but because it is vastly outnumbered.

The second law of thermodynamics states that the entropy of an isolated system never decreases. This is not a mysterious force. It is a statistical near-certainty: systems move toward states that can be realised in more ways, simply because there are more of them to land in. With 10²³ particles, "overwhelmingly probable" becomes indistinguishable from "certain".

This explains why time seems to have a direction. Most physical laws work identically forwards and backwards, but entropy does not. A cup smashing is ordinary; the fragments reassembling is not — not because it is impossible, but because it is one arrangement among an unimaginable number. This asymmetry is called the arrow of time, and it appears to trace back to the extremely low-entropy state of the early universe.

Why local order is fine. Life, crystals and buildings all reduce entropy locally — while increasing it more elsewhere, principally by dissipating heat. The total still rises. Earth is not an isolated system; it receives concentrated low-entropy sunlight and radiates diffuse heat.