Decay means to gradually break down, rot, or fall apart over time. When an apple sits too long on the counter, it begins to decay: its flesh turns brown and mushy, and eventually it might grow fuzzy mold. When a fallen tree decays in the forest, insects, fungi, and bacteria slowly break down the wood until it crumbles into rich soil.

Decay is a natural process that affects all organic matter. Dentists warn about tooth decay, which happens when bacteria and acids eat away at tooth enamel, creating cavities. In nature, decay plays a vital role: fallen leaves decay into nutrients that feed new plants, and decaying logs become homes for countless creatures while returning their materials to the ecosystem.

The word also describes things falling into disrepair through neglect. An abandoned house might fall into decay as its paint peels, its roof leaks, and its floorboards rot. A civilization or empire can decay when its institutions weaken and its influence fades.

Scientists use decay in specific ways too. Radioactive materials undergo radioactive decay, transforming into different elements over time. Sound waves decay as they travel, becoming quieter and fainter. In each case, decay captures that sense of something gradually diminishing, breaking down, or losing its original form and strength.