to become decomposed; rot: vegetation that was decaying.
to decline in excellence, prosperity, health, etc.; deteriorate.
Physics. (of a radioactive nucleus) to change spontaneously into one or more different nuclei in a process in which atomic particles, as alpha particles, are emitted from the nucleus, electrons are captured or lost, or fission takes place.
to cause to decay or decompose; rot: The dampness of the climate decayed the books.
decomposition; rot: Decay made the wood unsuitable for use.