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A specific place, marked by a dot and usually named with a letter.


A straight path that goes on forever in both directions. Named using two points on the line. A line has arrows on both ends that show it keeps going.

Line Segment

A part of a line that has two endpoints. A line segment is named using the two endpoints.


A ray has an endpoint on one side and keeps going forever on the other. You always name a Ray using the endpoint first.


A flat surface that keeps spreading in all directions forever. Must be named by three points on the plane.

Intersecting lines

Two lines that cross

Parallel lines

Two lines, that lie next to each other without ever getting close together or further apart.

Perpendicular lines

Two lines that make right angles when they cross.


The units used to measure an angle.

Right angle

An angle that measures 90 degrees.

Acute angle

An angle that measures less than 90 degrees. (Babies are small, babies are cute. An acute angle is small like a baby)

Obtuse angle

An angle that is more than 90 degrees. (Obtuse can also mean slow. An obtuse angle is a big, slow angle.)

Straight angle

An angle that measures 180 degrees. This is a flat angle. It looks like a line.


The vertex is the point where two rays or line segments meet to form an angle.

Equilateral Triangle

A triangle that has all three sides the same length.

Isosceles Triangle

A triangle with two sides that have the same length.

Scalene Triangle

A triangle with no equal length sides.

Right Triangle

A triangle that has a 90 degree, or right, angle.

Acute Triangle

A triangle that has three interior angles that are less than 90 degrees.

Obtuse Triangle

A triangle that has one interior angle that is greater than 90 degrees.

Interior Angle

An angle found on the inside of a shape or object.

Exterior Angle

An angle found on the outside of a shape or object.


Two shapes are congruent when all of the sides and angles of the two shapes are exactly the same. The object can be rotated and face a different direction, but the measurements must be the same.


The angles and the overall appearance of the shapes are the same, but one object is larger than the other.


Any shape that is a polygon and has four sides.


Any object made of at least three straight lines. There can be no gaps, no curves, and no lines that cross each other.


Opposite sides are parallel and congruent. Shape has four right angles.


All four sides are congruent and parallel. There are four right angles.


Four sided shape. Opposite sides are congruent and parallel.


A four sided shape with all sides congruent and opposite sides parallel.


A four sided shape with only one set of parallel lines.

Regular Polygon

A shape with congruent sides and identical angles.

Irregular Polygon

A shape made up of sides that are different links and unlike angles.


Five sided shape.


Six sided shape.


Seven sided shape.


Eight sided shape.


Nine sided shape.


Ten sided shape.


Eleven sided shape.


Twelve sided shape.


A line segment that joins two vertices of a shape by going through the middle of the shape.

Center (of a circle)

A point in the middle of a circle that is the same distance from outside of the circle all the way around.


A line segment that connects the center of the circle to a point on the circle.


A line segment that goes from one side of the circle to the other and passes through the center of the circle.


A line segment that goes from one side of the circle to the other side. A chord does not have to go through the center.

Central Angle

An angle, whose vertex is at the center of a circle.

Line Symmetry

If an object can have a line drawn down the center and both sides are congruent, then the object has line symmetry.

Rotational Symmetry

If you can rotate an object less than 360 degrees and the object looks like it did before you turned it, then the object has rotational symmetry.


The distance around the outside of a circle

Complementary Angle

two angles that add up to 90 degrees

Supplementary Angle

two angles whose measures have a sum of 180 degrees

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