To undergo decomposition or rot. The fallen leaves began to decay on the forest floor.
To decline in quality, power, or vigor. The old mansion started to decay after years of neglect.
To cause something to decompose or rot. The damp conditions in the basement will decay the wooden beams over time.
To undergo radioactive disintegration, emitting particles or radiation. A free neutron decays into a proton, an electron, and an antineutrino.