What are functions called that yield the same result when applied to one of their result values Suppose one of the following holds for all x, or all y, or all z:

f(f(x)) = f(x)
f(f(y)) = f(y)
f(f(f(z))) = f(f(z)) = f(z)


... how do you call such a function?
A function satisfying that property is called idempotent.