# Fiber (mathematics)

In mathematics, the term **fiber** (or **fibre** in British English) can have two meanings, depending on the context:

- In naive set theory, the
**fiber**of the element*y*in the set*Y*under a map*f*:*X*→*Y*is the inverse image of the singleton under*f*. - In algebraic geometry, the notion of a fiber of a morphism of schemes must be defined more carefully because, in general, not every point is closed.

## Definitions

### Fiber in naive set theory

Let *f* : *X* → *Y* be a map. The **fiber** of an element
commonly denoted by
is defined as

That is, the fiber of *y* under *f* is the set of elements in the domain of *f* that are mapped to *y*.

The inverse image or preimage
generalizes the concept of the fiber to subsets
of the codomain. The notation
is still used to refer to the fiber, as the fiber of an element *y* is the preimage of the singleton set
, as in
. That is, the fiber can be treated as a function from the codomain to the powerset of the domain:
while the preimage generalizes this to a function between powersets:

If *f* maps into the real numbers, so
is simply a number, then the fiber
is also called the **level set** of *y* under *f*:
If *f* is a continuous function and *y* is in the image of *f*, then the level set of *y* under *f* is a curve in 2D, a surface in 3D, and, more generally, a hypersurface of dimension *d* − 1.

### Fiber in algebraic geometry

In algebraic geometry, if *f* : *X* → *Y* is a morphism of schemes, the **fiber** of a point *p* in *Y* is the fiber product of schemes

where *k*(*p*) is the residue field at *p*.