cyclic points, on the plane completed by imaginary points at infinity
The two imaginary points at infinity whose homogeneous coordinates $(1,i,0)$ and $(1,-i,0)$ satisfy the equation of any circle. Straight lines passing through circular points are known as isotropic lines.
Circular points. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Circular_points&oldid=31662