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Related to cycloid: prolate cycloid, hypocycloid
click for a larger image
top: surface view of cycloid scales of a bony fish
bottom: The parametric equations of this cycloid are
x = r(θ - sin θ),
y = r(1 - cos θ).
click for a larger image


1. Resembling a circle.
2. Zoology
a. Relating to or being a kind of fish scale that is thin, rounded, and smooth-edged and often shows concentric growth rings.
b. Having cycloid scales.
3. Psychology Afflicted with or relating to cyclothymia.
n. Mathematics
The curve traced by a point on the circumference of a circle that rolls on a straight line.

[French cycloïde, from Greek kukloeidēs, circular : kuklos, circle; see kwel- in Indo-European roots + -oeidēs, -oid.]

cy·cloi′dal (-kloid′l) adj.


1. (Mathematics) resembling a circle
2. (Zoology) (of fish scales) rounded, thin, and smooth-edged, as those of the salmon
3. (Psychiatry) psychiatry (of a type of personality) characterized by exaggerated swings of mood between elation and depression. See also cyclothymia
4. (Mathematics) geometry the curve described by a point on the circumference of a circle as the circle rolls along a straight line. Compare trochoid1
5. (Zoology) a fish that has cycloid scales
cyˈcloidal adj
cyˈcloidally adv


(ˈsaɪ klɔɪd)

1. resembling a circle; circular.
a. (of the scale of a fish) smooth-edged and more or less circular in form.
b. (of a fish) having such scales.
3. Psychiatry. of or denoting a personality type characterized by wide fluctuations in mood within the normal range.
4. a curve generated by a point on the circumference of a circle that rolls, without slipping, on a straight line.
[1655–65; < Greek kykloeidḗs like a circle. See cycle, -oid]
cy•cloi′dal, adj.
ThesaurusAntonymsRelated WordsSynonymsLegend:
Noun1.cycloid - a line generated by a point on a circle rolling along a straight linecycloid - a line generated by a point on a circle rolling along a straight line
line roulette, roulette - a line generated by a point on one figure rolling around a second figure
curate cycloid - a cycloid generated by a point inside the rolling circle
prolate cycloid - a cycloid generated by a point outside the rolling circle
Adj.1.cycloid - resembling a circle
rounded - curving and somewhat round in shape rather than jagged; "low rounded hills"; "rounded shoulders"


[ˈsaɪklɔɪd] n (Geom) → cicloide f
References in classic literature ?
It was in the left hand try-pot of the Pequod, with the soapstone diligently circling round me, that I was first indirectly struck by the remarkable fact, that in geometry all bodies gliding along the cycloid, my soapstone for example, will descend from any point in precisely the same time.
In the first course, there was a shoulder of mutton cut into an equilateral triangle, a piece of beef into a rhomboides, and a pudding into a cycloid.
In brief - distinct grounds, and vivid circular or cycloid figures, of no meaning, are here Median laws.
The general morphology of the Tilapia is a rectangular body-shaped, covered with deep cycloid or cteniod scales.
v]) were defined by point (P) and intercept (I) counting using a cycloid grid and Stereology Toolbox (Morphometrix, Davis, CA) for a minimum of 200 points.
The parking actuator was based on the previous hybrid shift-by-wire system actuator, which features a combination of an SR motor and cycloid reduction gear.
Loaded tooth contact analysis of cycloid planetary gear drives, Proceedings of the 14th IFToMM2015 World Congress, Taipei, Taiwan, 953-960.
The functions of standard motions include harmonic, cycloid, modified harmonic, trapezoidal, modified trapezoidal, and polynomial.
Diagnostic features: compressed and fusiform body, oblique or upper mouth, prominent lower jaw, very small teeth arranged in villiform bands in the jaw, ventral gular plate between the two branches of the lower mandible, 32 to 36 gill rakers on the lower part of the first arch; 23 to 27 branchiostegals rays; simple and spineless dorsal fin with 13 to 16 rays, the last being a very long ray, anal fin with 21 to 25 soft rays, pectoral fins with 13 to 15 soft rays, pelvic fins with 10 or 11 soft rays; 37-42 large cycloid scales along the lateral line (Figure 1).
Body covered with small or moderate sized ctenoid scales, those of cheeks and opercles in 2-3 rows all cycloid except for occasional ctenoid scale posteriorly; lateral line scales simple.