As well as r.p.m. and r.p.s., the U.C.M. can be described using the speed with which the angle described by the radius which joins the centre of the movement to the body, changes. 

The way to express the units of speed of the UCM in the Système International d'Unités: that is, angular velocity, is radians per second.

Of course, all the ways used to express speed are related.  To understand this way of expressing angular velocity you have to know  what a radian is  .