1. The condition or process of being rotten or decomposition: "Brian was told that hardwood is more resistant to decays than softwood."

"Some fly larvae live in carrion and in animal flesh of varying stages of decay."

2. Structural or physical deterioration: "The old barn on Hugo's farm was rapidly falling into decay."

"Carnivorous larvae of some insects are produced in the decay of vegetation, in soil, and in rotten wood."

3. The process of declining in power, quality, or vigor: "The local minister reminded his church members to avoid moral decay in their lives."
4. A gradual decrease in the magnitude of a physical amount: "There is a possible decay of electrical fields in electromagnets."

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, the bridges leading across the river 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 tooth brushing, one's teeth will start to decay.
7. Declining from a state of normality, excellence, or prosperity; to deterioration: 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.

In alpha decay, an alpha particle (two protons and two neutrons) is emitted from a nucleus and the atomic number decreases by two to form a new nucleus.

An example would be when an atom of uranium isotope of mass 238, on emitting an alpha particle, becomes an atom of thorium, mass 234.

The radioactive decay of an atomic nucleus accompanied by emission of a beta particle.

Beta decay is the disintegration of the nucleus of an atom to produce a beta particle, or high-speed electron, and an electron-antineutrino.

During beta decay a neutron in the nucleus changes into a proton, thereby increasing the atomic number by one while the mass number stays the same.

The distribution in energy or momentum of the beta particles arising from a nuclear disintegration process.

A quantum transition between two energy levels of a nucleus in which a gamma ray is emitted.