*trochoi*

n. [ Pref. hypo- + trochoid. ] (Geom.) A curve, traced by a point in the radius, or radius produced, of a circle which rolls upon the concave side of a fixed circle. See Hypocycloid, Epicycloid, and Trochoid. [ 1913 Webster ]

n. [ Pref. retro- + choir. ] (Eccl. Arch.) Any extension of a church behind the high altar, as a chapel; also, in an apsidal church, all the space beyond the line of the back or eastern face of the altar. [ 1913 Webster ]

n. [ Gr. &unr_; a wheel + -oid; cf. F. trochoïde. 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. [ 1913 Webster ]

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. [ 1913 Webster ]

2. (Zool.) Top-shaped; having a flat base and conical spire; -- said of certain shells. [ 1913 Webster ]

3. (Zool.) Of or pertaining to the genus Trochus or family Trochidae. [ 1913 Webster ]