n. [ Cyclo- + -oid: cf. F. cycloïde. ] (Geom.) 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. [ 1913 Webster ]

☞ The common cycloid is the curve described when the generating point (p) is on the circumference of the generating circle; the curtate cycloid, when that point lies without the circumference; the prolate or inflected cycloid, when the generating point (p) lies within that circumference. [ 1913 Webster ]

a. (Zool.) Of or pertaining to the Cycloidei. [ 1913 Webster ]

Cycloid scale (Zool.), a fish scale which is thin and shows concentric lines of growth, without serrations on the margin. [ 1913 Webster ]

n. (Zool.) One of the Cycloidei. [ 1913 Webster ]

a. Pertaining to, or resembling, a cycloid; as, the cycloidal space is the space contained between a cycloid and its base. [ 1913 Webster ]

Cycloidal engine. See Geometric lathe. [ 1913 Webster ]

‖n. pl. [ NL., fr. Gr. ky`klos circle + -oid. ] (Zool.) An order of fishes, formerly proposed by Agassiz, for those with thin, smooth scales, destitute of marginal spines, as the herring and salmon. The group is now regarded as artificial. [ 1913 Webster ]

a. & n. (Zool.) Same as 2d and 3d Cycloid. [ 1913 Webster ]

n. [ Epicycle + -oid: cf. F. épicycloïde. ] (Geom.) A curve traced by a point in the circumference of a circle which rolls on the convex side of a fixed circle. [ 1913 Webster ]

☞ Any point rigidly connected with the rolling circle, but not in its circumference, traces a curve called an epitrochoid. The curve traced by a point in the circumference of the rolling circle when it rolls on the concave side of a fixed circle is called a hypocycloid; the curve traced by a point rigidly connected with the rolling circle in this case, but not its circumference, is called a hypotrochoid. All the curves mentioned above belong to the class class called roulettes or trochoids. See Trochoid. [ 1913 Webster ]

a. Pertaining to the epicycloid, or having its properties. [ 1913 Webster ]

Epicycloidal wheel, a device for producing straight-line motion from circular motion, on the principle that a pin fastened in the periphery of a gear wheel will describe a straight line when the wheel rolls around inside a fixed internal gear of twice its diameter. [ 1913 Webster ]

n. [ Pref. hypo- + cycloid: cf. F. hypocycloïde. ] (Geom.) A curve traced by a point in the circumference of a circle which rolls on the concave side in the fixed circle. Cf. Epicycloid, and Trochoid. [ 1913 Webster ]