between the particle's position vector and the positive 
-axis. [P2.7]
of: a particle in a plane, with respect to a point O, taken to be the origin of Cartesian coordinates in the plane
is: the angle between the particle's position vector and the positive -axis. [P2.7]
equivalently is: the polar angle of the point at which the particle is located, measured in a system of polar coordinates with origin at O.
Copyright 1997, The Open University