From Longman Dictionary of Contemporary Englishdecayde‧cay1 /dɪˈkeɪ/ ●●○ verb 1 [intransitive, transitive]DECAY to be slowly destroyed by a natural chemical process, or to make something do this Her body was already starting to decay. Most archaeological finds are broken, damaged, or decayed. decaying organic matterRegisterIn everyday English, people usually say rot rather than decay when talking about food:There was a smell of rotting vegetables.2 [intransitive]DECAY if buildings, structures, or areas decay, their condition gradually becomes worse Hundreds of historic buildings are being allowed to decay. Britain’s decaying inner cities3 [intransitive]LESS if traditional beliefs, standards etc decay, people do not believe in them or support them anymore syn decline In Orthodox Europe, mass religion seems to have decayed less.THESAURUSdecay to be slowly destroyed by a natural chemical process – use this especially about natural things such as wood or plants, or about teethThe leaves decay and enrich the soil.He had bad breath and decaying teeth.The fabric slowly began to decay. rot to decay. Rot is less formal than decay and is more common in everyday EnglishThe fruit was left to rot on the ground.rotting teethMost of the wood under the paint had rotted.the smell of rotting vegetation (=decaying leaves and plants)go off British English if food goes off, it starts to smell bad and is no longer be safe to eatI think the milk’s gone off.The meat smells as if it's gone off.spoil if food spoils, it starts to decay, so that it is no longer safe to eat. Spoil is more formal and is less common in everyday British English than go offFood left in the sun will quickly start to spoil.go mouldy British English, moldy American English to begin to have a soft green or black substance growing on the surface of the food, so that it is not good to eat anymoreUgh, the cheese has gone mouldy!decompose formal to decay – use this especially about dead plants or fleshleaves decomposing on the forest floorputrefy formal to decay and have a very bad smell – use this especially about flesh or plantsAfter two days, the body was already beginning to putrefy.putrefying meatbiodegrade to decay naturally into substances that do not harm the environment – use this especially about man-made materials and chemicalsUnlike many other materials, plastic does not biodegrade.→ See Verb table
Examples from the Corpusdecay• Freezing conditions will stop most things from decaying.• Now even elections seem to be decaying.• If you eat too many sweets, it'll make your teeth decay.• Some of the apples lying on the ground had already begun to decay.• A few species laid eggs beneath mounds of rotten vegetation that warmed as it decayed.• Bones signify the eternal being which does not decay.• There was now not the slightest doubt that Hsu was decaying and losing her structural integrity.• The decaying body of a man was found in a vacant warehouse.• It decays by a scheme that has two paths.• This decaying city was once the busiest port in the world.• However, objects decay despite our best efforts to conserve them.• the decaying moral values of American society• In a warm climate where flesh decays rapidly, there is more risk of infection from dead animals.• In one path, the 40K decays to a calcium isotope, 40Ca.• The buildings still stand, slowly decaying, without past or future, or any links with the surrounding land.