WordNet sense 1 (the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation):

disintegration, radioactive decay

WordNet sense 2 (a gradual decrease; as of stored charge or current):

decline

WordNet sense 3 (the organic phenomenon of rotting):

decomposition

WordNet sense 4 (lose a stored charge, magnetic flux, or current):

decompose, disintegrate

WordNet sense 5 (fall into decay or ruin):

crumble, delapidate