University Calculus: Early Transcendentals (3rd Edition)

Circle with equation $x^2+y^2=4$ in the $z=-2$ plane.
Here, the points are the intersection between the equation of cylinder $x^2+y^2=4$ and the plane $z=0$. The set of points that satisfies such equations has the equation of a circle lying in the xy plane, centered at origin $O$, having a circle of radius $2$ and shifted two points down from the xy plane.