A unit vector is a vector of length 1, sometimes also called a direction vector (Jeffreys and Jeffreys 1988). The unit vector having the same direction as a given (nonzero) vector is defined by
where denotes the norm of , is the unit vector in the same direction as the (finite) vector . A unit vector in the direction is given by
where is the radius vector.
When considered as the th basis vector of a vector space, a unit vector may be written (or ).