Gradual decline in strength, soundness, or prosperity or in degree of excellence or perfection.
Usually one encounters decay in both life and in death. While it’s easiest to observe decay in a physical world, you should definitely look for examples of it in a non-physical realm.
The decay of one’s mind is perhaps the most fascinating decay of a non-physical kind that is at the same time quite scary to witness. Easiest way to observe it is to look in a mirror every day. Look into your own eyes. See past the pigmented muscular curtain caught between your eye’s cornea and lens that we call the iris, see past past the optic nerve… The chill you feel is you witnessing gradual decay in real time. With each passing moment the cracks in your mind widen. Slowly but surely you are not only losing your mind but also yourself. Interesting and scary at the same time, right?
Also, in certain circumstances, decay can be beautiful…