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[ep-uh-sahy-kloid] /ˌɛp əˈsaɪ klɔɪd/
noun, Geometry.
a curve generated by the motion of a point on the circumference of a circle that rolls externally, without slipping, on a fixed circle. Equation: x = (a + b) cos(θ) − b cos[(a + b)θ/ b ] and y = (a + b) sin(θ) − b sin[(a + b)θ/ b ].
Compare epitrochoid.
Origin of epicycloid
First recorded in 1780-90; epicycle + -oid
Related forms
epicycloidal, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2017.
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British Dictionary definitions for epicycloid


the curve described by a point on the circumference of a circle as this circle rolls around the outside of another fixed circle, the two circles being coplanar Compare hypocycloid, cycloid (sense 4)
Derived Forms
epicycloidal, adjective
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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epicycloid in Science
The curve described by a point on the circumference of a circle as the circle rolls on the outside of the circumference of a second, fixed circle.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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