The position of a particle refers to its location in the space at a certain moment of time. In general the position is measured by a vector joining a fixed point (known as origin) to the moving particle. This vector is known as Position vector.
In two dimensions, either cartesian or polar coordinates may be used, and the use of unit vectors is common. A position vector r may be expressed in terms of the unit vectors.
In three dimensions, cartesian or spherical polar coordinates are used, as well as other coordinate systems for specific geometries.