Decay
Etymology
From Old North French decair, from Late Latin decadere, "to fall away, fall off," from de-, "down," plus cadere, "to fall" (the same root behind "cadence" and "decadence"). The image is of something dropping away piece by piece from its former state. English applies it to matter, health, and civilizations alike.
noun · /dih-KAY/
The gradual decline, rotting, or falling apart of something once whole or thriving.
What it names
The slow downward process by which a body, structure, or institution loses its integrity over time, whether by rot, neglect, or the ordinary wearing-out of things.
Example
You could read the town’s decay in its main street: half the shopfronts boarded, the rest selling off their stock.
Related idea
Entropy is the physicist’s formal measure of disorder increasing; decay is the visible, everyday face of that decline in a specific thing you can point to.
noun / verb · /dih-KAY/
In physics, the spontaneous breakdown of an unstable atomic nucleus or particle, giving off radiation as it transforms.
What it names
radioactive decay — the process measured by a substance’s half-life
Example
Carbon-14 decays at a known rate, which is what makes radiocarbon dating possible.
Related idea
The rotting sense and the nuclear one share the idea of a thing falling apart on its own over time — but decay here is exact and clock-like, the engine behind measuring deep time, not mere drift into entropy.