† focoidn.
Geometry. Obsolete. rare.
Either of the two points on the line at infinity through which, in projective geometry, every circle passes.
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ideal point1879
focoid1881
1881 C. Taylor Anc. & Mod. Geom. Conics 308 The two focoids or circular points at infinity.
1920 Nature 5 Aug. 708/2 The device should be classed with the focoids, those two imaginary points at infinity where any two concentric circles touch.
This entry has been updated (OED Third Edition, September 2016; most recently modified version published online June 2018).
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n.1881