A circle passing through the pitch point with its center at the center of cam axis is known as

Imagine a cam, that rotating part that makes other parts move up and down or back and forth.
It has a circular shape, but only a part of it is important for the actual movement.
This important part is called the pitch circle.
Now, imagine drawing a circle that goes through the pitch point (the point where the cam and the follower first touch).
And imagine that the center of this circle is exactly the same as the center of the cam's axis of rotation.
That special circle is called the prime circle.
So, a circle passing through the pitch point with its center at the center of cam axis is known as a prime circle.