Let's say that we have an ordinary function f : X -> Y. If we pick out one y from the target or the codomain Y, then it either has x's from the source or the domain X that are assigned, or mapped, or sent to it by f or it doesn't. Let's say it does. Then here is the picture of what I will frequently call the "fiber (of f) at y": | Let's say that we have an ordinary function f : X -> Y. If we pick out one y from the target or the codomain Y, then it either has x's from the source or the domain X that are assigned, or mapped, or sent to it by f or it doesn't. Let's say it does. Then here is the picture of what I will frequently call the "fiber (of f) at y": |