Orthogonal circles
In geometry, two circles are said to be orthogonal if their respective tangent [lines to circles|tangent line]s at the points of intersection are perpendicular.
A straight line through a circle's center is orthogonal to it, and if straight lines are also considered as a kind of generalized circles, for instance in inversive geometry, then an orthogonal pair of lines or line and circle are orthogonal generalized circles.
In the Poincaré [disk model|conformal disk model] of the hyperbolic plane, every geodesic is an arc of a generalized circle orthogonal to the circle of ideal points bounding the disk.