radian The size of an angle at the center of a circle formed by two radii and an arc that is the length of the radius more info
radius A line joining the center of a circle to a point on the circle's circumference.
range In statistics, the difference between the greatest and least values in a set of data
ratio A ratio is a comparison of two similar quantities obtained by dividing one quantity by the other. Ratios are written with the : symbol. more info
rational number Numbers that can be written as a/b, where a and b are integers and b is not equal to 0. Real numbers that can be expressed as terminating or repeating decimals
rectangle A quadrilateral with opposite sides equal and parallel and containing four right angles (a square is a rectangle).
reflex angle An angle greater than 180º.
region The interior area enclosed by a perimeter (the area outside the perimeter is the exterior region).
regular shape A polygon is regular if all its sides and angles are congruent (opposite - irregular).
rhombus A parallelogram with congruent sides and containing no right angles (a "diamond" shape).
right angle An angle containing 90º.
right-angled triangle A triangle containing one right angle (90º).
rotating Turning in a clockwise or counter-clockwise direction.
