cycloid (ˈsaɪklɔɪd)  
—adj  
1.  resembling a circle 
2.  (of fish scales) rounded, thin, and smoothedged, as those of the salmon 
3.  psychiatry See also cyclothymia (of a type of personality) characterized by exaggerated swings of mood between elation and depression 
—n  
4.  geometry Compare trochoid the curve described by a point on the circumference of a circle as the circle rolls along a straight line 
5.  a fish that has cycloid scales 
cy'cloidal  
—adj  
cy'cloidally  
—adv 
cycloid (sī'kloid') Pronunciation Key
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cycloid
the curve generated by a point on the circumference of a circle that rolls along a straight line. If r is the radius of the circle and theta (theta) is the angular displacement of the circle, then the polar equations of the curve are x = r(theta  sin theta) and y = r(1  cos theta)
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