Withering is the process of becoming weaker. You can watch the withering of a flower bouquet in water — over the days and weeks, the blossoms become droopy and faded.
You might talk about the withering of trees in a drought or the withering of people's bank accounts during an economic recession. Withering can also be used as an adjective, to describe something with a terrible, destructive strength, like a withering hurricane. It comes from the verb wither, to dry up or shrivel, from the Old English word wydderen, which is a variation on wederen, "to expose to the weather."