trochoid

Trochoid \Tro"choid\, n. [Gr. ? a wheel + -oid; cf. F.
   trocho["i]de. See {Troche}.] (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.
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Trochoid \Tro"choid\, 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.
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   2. (Zool.) Top-shaped; having a flat base and conical spire;
      -- said of certain shells.
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   3. (Zool.) Of or pertaining to the genus Trochus or family
      {Trochidae}.
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