geom
61reentering angle — Geom. an interior angle of a polygon that is greater than 180°. Also called reentrant angle. See diag. under salient. [1690 1700] * * * …
62reflex angle — Geom. an angle greater than 180° and less than 360°. [1895 1900] * * * …
63Riemannian geometry — Geom. 1. Also called elliptic geometry. the branch of non Euclidean geometry that replaces the parallel postulate of Euclidean geometry with the postulate that in a plane every pair of distinct lines intersects. Cf. hyperbolic geometry. 2. the… …
64right circular cone — Geom. a cone whose surface is generated by lines joining a fixed point to the points of a circle, the fixed point lying on a perpendicular through the center of the circle. Cf. oblique circular cone. See illus. under cone. [1940 45] * * * …
65right circular cylinder — Geom. a cylinder generated by the revolution of a rectangle about one of its sides. Cf. oblique circular cylinder. [1940 45] * * * …
66ruled surface — Geom. a surface that can be generated by a straight line, as a cylinder or cone. [1860 65] * * * …
67skew lines — Geom. any lines in space that do not intersect and are not parallel. [1950 55] * * * …
68slant height — Geom. (of a right circular cone) the distance from the vertex to any point on the circumference of the base. [1790 1800] * * * …
69solid angle — Geom. an angle formed by three or more planes intersecting in a common point or formed at the vertex of a cone. [1695 1705] * * * …
70spherical angle — Geom. an angle formed by arcs of great circles of a sphere. [1670 80] * * * …
