Biology To break down into component parts; rot. Physics To disintegrate or diminish by radioactive decay. Electronics To decrease gradually in magnitude. Used of voltage or current. Aerospace To decrease in orbit. Used of an artificial satellite. To fall into ruin: a civilization that had begun to decay. Pathology To decline in health or vigor; waste away. To decline from a state of normality, excellence, or prosperity; deteriorate. To cause to decay. The destruction or decomposition of organic matter as a result of bacterial or fungal action; rot. Rotted matter. Physics Radioactive decay. Aerospace The decrease in orbital altitude of an artificial satellite as a result of conditions such as atmospheric drag. A gradual deterioration to an inferior state: tooth decay; urban decay. A falling into ruin.