1. resembling a circle; circular.
  2. (of the scale of a fish) smooth-edged, more or less circular in form, and having concentric striations.
  3. (of a fish) having such scales.
  4. Psychiatry. of or noting a personality type characterized by wide fluctuation in mood within the normal range.
  1. a cycloid fish.
  2. 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 formscy·cloi·dal, adjectivecy·cloi·dal·ly, adverb Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for cycloid

Historical Examples of cycloid

  • It is not yet known to what philosopher we owe the invention of the cycloid.

  • Pascal is said to have written his treatise on the cycloid from a religious motive.

    Sir Christopher Wren

    Lucy Phillimore

  • So that it is a kind of Cycloid, of which, however, the points can be found geometrically.

    Treatise on Light

    Christiaan Huygens

  • Rather it travels along a cycloid, bending back upon itself, following the movement of man.

  • In brief—distinct grounds, and vivid circular or cycloid figures, of no meaning, are here Median laws.

British Dictionary definitions for cycloid


  1. resembling a circle
  2. (of fish scales) rounded, thin, and smooth-edged, as those of the salmon
  3. psychiatry (of a type of personality) characterized by exaggerated swings of mood between elation and depressionSee also cyclothymia
  1. geometry the curve described by a point on the circumference of a circle as the circle rolls along a straight lineCompare trochoid (def. 1)
  2. a fish that has cycloid scales
Derived Formscycloidal, 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

cycloid in Science


  1. Resembling a circle.
  2. Thin, rounded, and smooth-edged, like a disk. Used of fish scales.
  3. 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.