In mathematics, a unit vector in a normed vector space is a vector (often a spatial vector) whose length is 1 (the unit length). A unit vector is often denoted by a lowercase letter with a "hat", like this: (pronounced "i-hat").
The normalized vector or versor of a non-zero vector u is the unit vector codirectional with u, i.e.,
where ||u|| is the norm (or length) of u. The term normalized vector is sometimes used as a synonym for unit vector.
The elements of a basis are usually chosen to be unit vectors. Every vector in the space may be written as a linear combination of unit vectors. The most commonly encountered bases are Cartesian, polar, and spherical coordinates. Each uses different unit vectors according to the symmetry of the coordinate system. Since these systems are encountered in so many different contexts, it is not uncommon to encounter different naming conventions than those used here.
Orthogonal coordinates 
Cartesian coordinates 
These are often written using normal vector notation (e.g. i, or ) rather than the circumflex notation, and in most contexts it can be assumed that i, j, and k, (or and ) are versors of a Cartesian coordinate system (hence a term of mutually orthogonal unit vectors). The notations , , , or , with or without hat/circumflex, are also used, particularly in contexts where i, j, k might lead to confusion with another quantity (for instance with index symbols such as i, j, k, used to identify an element of a set or array or sequence of variables). These vectors represent an example of a standard basis.
When a unit vector in space is expressed, with Cartesian notation, as a linear combination of i, j, k, its three scalar components can be referred to as direction cosines. The value of each component is equal to the cosine of the angle formed by the unit vector with the respective basis vector. This is one of the methods used to describe the orientation (angular position) of a straight line, segment of straight line, oriented axis, or segment of oriented axis (vector).
Cylindrical coordinates 
The unit vectors appropriate to cylindrical symmetry are: (also designated or ), the distance from the axis of symmetry; , the angle measured counterclockwise from the positive x-axis; and . They are related to the Cartesian basis , , by:
It is important to note that and are functions of , and are not constant in direction. When differentiating or integrating in cylindrical coordinates, these unit vectors themselves must also be operated on. For a more complete description, see Jacobian matrix. The derivatives with respect to are:
Spherical coordinates 
The unit vectors appropriate to spherical symmetry are: , the direction in which the radial distance from the origin increases; , the direction in which the angle in the x-y plane counterclockwise from the positive x-axis is increasing; and , the direction in which the angle from the positive z axis is increasing. To minimize degeneracy, the polar angle is usually taken . It is especially important to note the context of any ordered triplet written in spherical coordinates, as the roles of and are often reversed. Here, the American "physics" convention is used. This leaves the azimuthal angle defined the same as in cylindrical coordinates. The Cartesian relations are:
The spherical unit vectors depend on both and , and hence there are 5 possible non-zero derivatives. For a more complete description, see Jacobian. The non-zero derivatives are:
General unit vectors 
|Tangent vector to a curve/flux line||
A normal vector to the plane containing and defined by the radial position vector and angular tangential direction of rotation is necessary so that the vector equations of angular motion hold.
|Normal to a surface tangent plane/plane containing radial position component and angular tangential component||
In terms of polar coordinates;
|Binormal vector to tangent and normal|||
|Parallel to some axis/line||
One unit vector aligned parallel to a principle direction (red line), and a perpendicular unit vector is in any radial direction relative to the principle line.
|Perpendicular to some axis/line in some radial direction|
|Possible angular deviation relative to some axis/line||
Unit vector at acute deviation angle φ (including 0 or π/2 rad) relative to a principle direction.
Curvilinear coordinates 
In general, a coordinate system may be uniquely specified using a number of linearly independent unit vectors equal to the degrees of freedom of the space. For ordinary 3-space, these vectors may be denoted . It is nearly always convenient to define the system to be orthonormal and right-handed:
See also 
- Unit interval
- Unit square, cube, circle, and sphere
- Cartesian coordinate system
- Polar coordinate system
- Coordinate system
- Curvilinear coordinates
- Right versor
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