Pronunciation: (ep"u-sī'kloid),[key] —n.Geom. a curve generated by the motion of a point on the circumference of a circle that rolls externally, without slipping, on a fixed circle. Equation: x = (a + b) cos θ - b cos [(a + b)θ/b] and y = (a + b) sin θ - b sin [(a + b)θ/b]. Cf. epitrochoid.