Math. Also -oide. [ad. Gr. καρδιοειδής heart-shaped, f. καρδία + εἶδος form.] A curve somewhat resembling a heart in shape.
If through a fixed point in the circumference of a circle straight lines be drawn, intersecting the circle at different points, and such that the length of each line on each side of the point of intersection is equal to the diameter of the circle, the extremities of these lines will trace out a cardioid, having its cusp at the fixed point. The cardioid is also traced out by a point in the circumference of one circle rolling round the circumference of another equal to it.
1753. in Chambers, Cycl. Supp., The Cardioide.
1852. Salmon, Higher Pl. Curve (1879), II. 44. The curve is of the form of a heart and is called the cardioide.
1879. Thomson & Tait, Nat. Phil., I. I. § 94. We have … the case of a circle rolling externally on another of equal size. The curve in this case is called the Cardioid.
