Trochoid

n.	1.	(Geom.) The curve described by any point in a wheel rolling on a line; a cycloid; a roulette; in general, the curve described by any point fixedly connected with a moving curve while the moving curve rolls without slipping on a second fixed curve, the curves all being in one plane. Cycloids, epicycloids, hypocycloids, cardioids, etc., are all trochoids.
a.	1.	(Anat.) Admitting of rotation on an axis; - sometimes applied to a pivot joint like that between the atlas and axis in the vertebral column.
	2.	(Zool.) Top-shaped; having a flat base and conical spire; - said of certain shells.
	3.	(Zool.) Of or pertaining to the genus Trochus or family Trochidæ.