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Geometry vocabulary

Review nine core geometry terms. Compare terms that are easy to mix up, including area and perimeter, or radius and diameter.

Word	Meaning	In a sentence
anglenoun	A figure formed by two rays that start at the same point, or the measure of the turn between them.	The two rays form a right angle.
areanoun	The amount of two-dimensional space inside a region or across a surface.	A rectangle that is 3 meters by 4 meters has an area of 12 square meters.
circumferencenoun	The distance around a circle.	We wrapped a string around the lid to estimate its circumference.
diameternoun	A line segment across a circle through its center, or the length of that segment.	Draw a diameter through the center of the circle.
paralleladjective	Describing straight lines in the same plane that never meet, however far they extend.	Opposite sides of a rectangle are parallel.
perimeternoun	The total distance around the boundary of a flat shape.	The square has sides of 4 centimeters and a perimeter of 16 centimeters.
polygonnoun	A closed, flat shape with three or more straight sides.	A triangle is a polygon with three sides.
radiusnoun	A line segment from a circle's center to its boundary, or the length of that segment.	The radius of a circle with diameter 12 centimeters is 6 centimeters.
rectanglenoun	A flat, four-sided shape with four right angles.	Draw a rectangle that is six centimeters long and three centimeters wide.