To decay is to slowly break down, rot, or fall apart over time, often because of natural processes. It can also describe the gradual decline of quality, power, or strength.
It comes from Old French “dechëoir,” from Latin “decidere,” meaning “to fall down, fall off.” The sense shifted from literally falling away to gradually breaking down.
Decay is nature’s recycling system—what looks like destruction is actually preparation for new growth. The same word describes rotting fruit and the slow decline of empires, which hints at how similar those processes really are.
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