[ sahy-kloid ]
/ ˈsaɪ klɔɪd /


resembling a circle; circular.
(of the scale of a fish) smooth-edged, more or less circular in form, and having concentric striations.
(of a fish) having such scales.
Psychiatry. of or noting a personality type characterized by wide fluctuation in mood within the normal range.


a cycloid fish.
Geometry. a curve generated by a point on the circumference of a circle that rolls, without slipping, on a straight line.

Origin of cycloid

First recorded in 1655–65, cycloid is from the Greek word kykloeidḗs like a circle. See cycl-, -oid

Related forms

cy·cloi·dal, adjectivecy·cloi·dal·ly, adverb Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for cycloid

British Dictionary definitions for cycloid


/ (ˈsaɪklɔɪd) /


resembling a circle
(of fish scales) rounded, thin, and smooth-edged, as those of the salmon
psychiatry (of a type of personality) characterized by exaggerated swings of mood between elation and depressionSee also cyclothymia


geometry the curve described by a point on the circumference of a circle as the circle rolls along a straight lineCompare trochoid (def. 1)
a fish that has cycloid scales

Derived Forms

cycloidal, adjectivecycloidally, adverb
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

Science definitions for cycloid


[ sīkloid′ ]

Resembling a circle.
Thin, rounded, and smooth-edged, like a disk. Used of fish scales.
The curve traced by a point on the circumference of a circle that rolls on a straight line.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.