Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. A curve traced by a point on or connected with a circle as the circle rolls along a fixed straight line.
  • adj. Capable of or exhibiting rotation about a central axis.
  • adj. Permitting rotation, as a pulley or pivot.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. The curve traced by a point on a circle as it rolls along a straight line
  • adj. capable of rolling
  • adj. allowing rotation

from the GNU version of the Collaborative International Dictionary of English

  • 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.
  • adj. Admitting of rotation on an axis; -- sometimes applied to a pivot joint like that between the atlas and axis in the vertebral column.
  • adj. Top-shaped; having a flat base and conical spire; -- said of certain shells.
  • adj. Of or pertaining to the genus Trochus or family Trochidæ.

from The Century Dictionary and Cyclopedia

  • n. In geometry: The curve described by any point on a radius of the rolling circle, or on a radius produced when two circles are tangent either externally or internally and, while one of them remains fixed, the other rolls upon it without sliding.
  • In geometry, trochoidal.
  • In anatomy, rotating or revolving like a wheel; pivotal, as an articulation; trochoidal: applied to that kind of rotatory arthrosis in which a part revolves to some extent upon another, as the head of the radius in the lesser sigmoid cavity of the ulna in pronation and supination of the forearm, or the atlas about the odontoid process of the axis in shaking the head.
  • In conchology, top-shaped, like a shell of the genus Trochus; conical with a flat base; of or related to the Trochidæ.
  • n. In geometry, a prolate or curtate cycloid or curve traced by a point in fixed connection with, but not generally on the circumference of, a wheel which rolls upon a right line. If the point is outside the circumference, the trochoid has loops; if inside, it has waves. See cycloid.
  • n. In anatomy, a rotatory or pivotal joint; diarthrosis rotatorius; cyclarthrosis.
  • n. In conchology, a top-shell, or some similar shell; any member of the Trochidæ.

Etymologies

Greek trokhoeidēs, wheellike : trokhos, wheel; see trochee + -oeidēs, -oid.
(American Heritage® Dictionary of the English Language, Fourth Edition)
From Ancient Greek τροχοειδής (trokhoeidēs), from τροχός (trokhos, "wheel") + εἶδος (eidos, "form, image") (Wiktionary)

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