# Extensive Definition

A hyperplane is a concept in geometry. It is a
higher-dimensional generalization of the concepts of a line in
Euclidean
plane geometry and a plane in
3-dimensional Euclidean geometry. The most familiar kinds of
hyperplane are affine
and linear
hyperplanes; less familiar is the projective
hyperplane.

In a one-dimensional space (a straight line), a
hyperplane is a point;
it divides a line into two rays.
In two-dimensional space (such as the xy plane), a hyperplane is a
line;
it divides the plane into two half-planes. In
three-dimensional space, a hyperplane is an ordinary plane;
it divides the space into two half-spaces.
This concept can also be applied to four-dimensional space and
beyond, where the dividing object is simply referred to as a
"hyperplane".

## Affine hyperplanes

In the general case, an affine hyperplane is an affine subspace of codimension 1 in an affine geometry. In other words, a hyperplane is a higher-dimensional analog of a (two-dimensional) plane in three-dimensional space.An affine hyperplane in n-dimensional space with
coordinates in a field
K can be described by a non-degenerate linear
equation of the following form:

- a1x1 + a2x2 + ... + anxn = b.

Here, non-degenerate means that not all the ai
are zero. If b=0, one obtains a linear or homogeneous hyperplane,
which goes through the origin of the coordinate system.

The two half-spaces defined by a hyperplane in
n-dimensional space with real-number coordinates are:

- a1x1 + a2x2 + ... + anxn ≤ b

and

- a1x1 + a2x2 + ... + anxn ≥ b.

## Linear hyperplanes

In linear algebra the term "hyperplane" is used
in a more limited way. A hyperplane in a vector space
is a vector subspace (or "linear subspace") whose dimension is 1
less than that of the whole vector space. These hyperplanes are the
affine hyperplanes that contain the origin of coordinates.

## Projective hyperplanes

There are also projective hyperplanes, in
projective geometry. Projective geometry can be viewed as affine
geometry with vanishing
points (points at infinity) added. An affine hyperplane
together with the associated points at infinity forms a projective
hyperplane. There is one other projective hyperplane: the set of
all points at infinity, called the infinite or ideal
hyperplane.

In real projective space, a hyperplane does not
divide the space into two parts; rather, it takes two hyperplanes
to separate points and divide up the space.

## Notes

- Hyperplanes in complex affine space do not divide the space into two parts. For this property, the coordinate field has to be an ordered field.
- The term realm has been proposed for a three-dimensional hyperplane in four-dimensional space, but it is used rarely, if ever.

## See also

hyperplane in Bosnian: Hiperravan

hyperplane in German: Hyperebene

hyperplane in Spanish: Hiperplano

hyperplane in Esperanto: Hiperebeno

hyperplane in French: Hyperplan

hyperplane in Italian: Iperpiano

hyperplane in Polish: Hiperpłaszczyzna

hyperplane in Portuguese: Hiperplano

hyperplane in Russian: Гиперплоскость

hyperplane in Serbian: Хиперраван

hyperplane in Chinese: 超平面