Decaying

To deteriorate, to get worse, to lose strength or health, to decline in quality.

"The pair loved to take pictures in the decaying hospital on forty-third street."

(of organic material) To rot, to go bad.

"The cat's body decayed rapidly."

(of an unstable atom) To change by undergoing fission, by emitting radiation, or by capturing or losing one or more electrons.

(of a quantum system) To undergo optical decay, that is, to relax to a less excited state, usually by emitting a photon or phonon.

The process of undergoing decay.