(追記) (追記ここまで)
Cardioid
A curve that is somewhat heart shaped. A cardioid can be drawn by tracing the path of a point on a circle as the circle rolls around a fixed circle of the same radius. The equation is usually written in polar coordinates.
Note: A cardioid is a special case of the limaçon family of curves.
Cardioid: r = a ± a cos θ (horizontal) or r = a ± a sin θ (vertical)
See also