1. To break down into component parts; rot: Gerda put the vegetable peelings in the compost pile so they can decay and create fertilizer that can be used in her garden.
2. In physics, to disintegrate or diminish by a radioactive process: The spent nuclear waste will decay in the underground storage tanks, according to the latest news reports from the government.
3. In electronics, to decrease gradually in magnitude with reference to voltage or current: During what are called "Brown Outs", the quality of light in the city's core actually decays due to the electrical circuits being overloaded.
4. In aerospace, to decrease in orbit; referring to an artificial man-made apparatus that orbits around the Earth: Towards the end of a satellite's life span in space, the orbit begins to decay and it appears to circle closer to the Earth's atmosphere.
5. To fall into ruin: Without good repairs to the infrastructure, bridges crossing rivers into cities will decay and fall apart.
6. In pathology, to decline in health or vigor; to waste away: The dentist cautioned that without regular teeth brushing, one's teeth will start to decay.
7. To decline from a state of normality, excellence, or prosperity; to deteriorate: There are days when Dora's mother is convinced that her memory is beginning to decay because she can't seem to remember specific dates, names, etc. as well as she could before.