director circle
curve consisting of all points where two perpendicular tangent lines to the ellipse or hyperbola cross each other
en-US/diˈɹɛktɚ ˈsɝkəl/
en-GB/dɪˈɹɛktə ˈsɜːkəl/
curve consisting of all points where two perpendicular tangent lines to the ellipse or hyperbola cross each other