1. (n.) 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.

2. (a.) Admitting of rotation on an axis; -- sometimes applied to a pivot joint like that between the atlas and axis in the vertebral column.

3. (a.) Top-shaped; having a flat base and conical spire; -- said of certain shells.

4. (a.) of or pertaining to the genus Trochus or family Trochidae.