Radius - A line segment from the center of a circle to any point on the circle. Or the line from the center of a spere to any point on the outside edge of the sphere. The radius is the distance from the center of a circle/sphere to the outside edge.
Ratio - The relation between to quantities. Ratios can be expressed in words, fractions, decimals or percents. E.g., the ratio given when a team wins 4 out of 6 games can be said a 4:6 or four out of six or 4/6.
Ray - A straight line with one endpoint. The line extends infinitely.
Range - The difference between the maximum and the minimum in a set of data.
Rectangle - A parallelogram which has four right angles.
Repeating Decimal - A decimal with endlessly repeating digits. E.g., 88 divided by 33 will give a 2.6666666666666
Reflection - A mirror image of a shape or an object. Obtained from flipping the image/object.
Remainder - The number that is left over when the number cannot be divided evenly into the number.
Right Angle - An angle that is 90°.
Right Triangle - A triangle having one angle equal to 90°.
Rhombus - A parallelogram with four equal sides, sides are all the same length.