Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 adj. Resembling a circle.
 adj. Zoology Thin, rounded, and smoothedged; disklike. Used of fish scales.
 adj. Zoology Having or composed of such scales.
 adj. Psychiatry 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.
 n. Zoology A fish having cycloid scales.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. The locus of a point on the circumference of a circle that rolls without slipping on a fixed straight line.
 n. A fish having cycloid scales.
 adj. Resembling a circle; cycloidal.
 adj. (of fish scales) Thin and rounded, with smooth edges.
from the GNU version of the Collaborative International Dictionary of English
 adj. Of or pertaining to the Cycloidei.
 n. A curve generated by a point in the plane of a circle when the circle is rolled along a straight line, keeping always in the same plane.
 n. One of the Cycloidei.
from The Century Dictionary and Cyclopedia
 Resembling a circle; having a circular form.
 Specifically In ichthyology: More or less circular, with concentric striations: applied to the scales of certain fishes. See cut under scale.
 Having somewhat circular scales, as a fish; specifically, pertaining to the Cycloidei.
 n. A curve generated by a point in the circumference or on a radius of a circle when the circle is rolled along a straight line and kept always in the same plane.
 n. In ichthyology, a cycloid fish; a fish with cycloid scales, or one of the Cycloidei.
 In chem., containing a cycle or ring of atoms: used especially of the structure of organic compounds.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 adj. resembling a circle
 n. a line generated by a point on a circle rolling along a straight line
Etymologies
Examples

8.13 A Beautiful Curve ∗ One of the most wonderful curves I can think of, and one that had a great in ﬂ uence on me in my youth, is called a cycloid, which is the locus ∗ ∗ of a ﬁ xed point on the circumference of a circle as it rolls, without slipping, along a straight line (Figure 8.8).

A cycloid is a curve traced by a point in the circumference of a wheel when the wheel is rolled along in a straight line.

Newton solved it correctly; he showed that the curve was a part of what is termed a cycloid  that is to say, a curve like that which is described by a point on the rim of a carriagewheel as the wheel runs along the ground.

Thin, translucent, and lacking enamel as well as dentine, these modern structures are known as cycloid and ctenoid scales.

The finegrained assemblage is dominated by tabular, lowdensity elements, such as cycloid scales and fish vertebrae.

A curve described in space by a point of a circle or sphere, which itself is carried along at the same time, is some kind of cycloid; if the centre of the tracing circle travels along a straight line, we get the ordinary cycloid, the curve traced in air by a nail on a coachwheel; but if the centre of the tracing circle be carried round another circle the curve described is called an epicycloid.

He published works on the arithmetical triangle, on wagers and the theory of probabilities, and on the roulette or cycloid.

It is the first of the three hypotheses from which Huygens develops his theory of “falling heavy bodies and their motion in a cycloid” in his Horologium Oscillatorium of 1673: If there were no gravity, and if the air did not impede the motion of bodies, then any body will continue its given motion with uniform velocity in a straight line.

Another is the famous brachistochrone problem in which a ball rolls down a curve a cycloid that gives the minimum time of travel.

Lips.an. 1695 — to these a lead weight is an eternal balance, and keeps watch as well as a couple of centinels, inasmuch as the construction of them was a curve line approximating to a cycloid — if not a cycloid itself.
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duckbill commented on the word cycloid
Used in the sense of 'manicdepressive' in some literature.
April 24, 2011