Geometry Terms for 4th grade List of basic geometry terms Tools A B angle Figure formed by 2 rays or 2 line segments with a common endpoint vertex point where the rays of an angle or the sides of a polygon meet acute angle measures between 0 and 90 degrees obtuse angle measures between 90 and 180 degrees reflex angle measures between 180 and 360 degrees right angle measures 90 degrees straight angle measures 180 degrees concave polygon polygon with at least one vertex "pushed in" convex polygon polygon in which all vertices are "pushed outward" face a flat surface on a 3D shape heptagon polygon with 7 sides hexagon polygon with 6 sides intersecting lines lines that meet or cross each other kite a quadrilateral with 2 pairs of adjacent equal sides line a straight path that extends infinitely in opposite directions octagon polygon with 8 sides parallel lines lines that never meet and always the same distance apart parallelogram quadrilateral with 2 pairs of parallel sides perpendicular meeting at right angles point exact location in space rectangle a parallelogram with 4 right angles rhombus a quadrilateral whose sides are all the same length trapezoid a quadrilateral that has exactly one pair of parallel sides triangle polygon with 3 sides square a rectangle with all sides equal ray a straight path that extends infinitely from its endpoint line segment part of a line