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Undefined term
Words that are not explained by using more basic words. The basic undefined terms of geometry are point, line, and plane.

Point
A location. (point is an undefined term)
 point A

Line
A collection of point, with no thickness, that extends forever in two opposite directions. (line is an undefined term)

Plane
A flat surface, with no thickness, that extends forever in all directions (plane is an undefined term)

Collinear Points
Points that lie on the same line.

Coplanar
Points and lines in the same plane.
 A, B, and C are coplanar

Intersection
The set of all points that are common to two or more geometric figures.

Space
A boundless threedimensional set of all points.

Line segment
Part of a line consisting of two endpoints and all points between them.

Congruent segments
Segments that have the same length.

Construction
A geometric figure created using a straightedge and a compass.

Distance (between two points)
The length of the segment between two points.

Irrational number
A number that cannot be expressed as the quotient of two integers.

Midpoint
A point that divides a segment into two congruent segments.
B is the midpoint of segment AC.


A segment, line, or plane that intersects a segment at its midpoint.

Ray
Part of a line consisting of one endpoint and all the points of the line on one side of the endpoint.

Opposite Rays
Two collinear rays with the same endpoint.

Angle
An object formed by two rays with the same endpoint.

Vertex
The point of intersection of two rays.

Right Angle
An angle with a measure equal to 90 degrees.

Acute Angle
An angle with a measure less than 90 degrees.

Obtuse Angle
An angle with a measure greater than 90 degrees and less than 180 degrees.

Straight Angle
An angle with degree measure equal to 180.

Angle Bisector
A ray that divides an angle into two congruent coplanar angles.

Adjacent angles
Two coplanar angles with a common side, a common vertex, and no common interior points.

Linear pair
A pair of adjacent angles whose nonshared rays form a line.

Vertical Angles
Two nonadjacent angles formed by two intersecting lines.

Supplementary Angles
Two angles whose measures have a sum of 180 degrees.

Complementary Angles
Two angles whose measures have a sum of 90 degrees.

Perpendicular Lines
Two lines that intersect to form right angles.

Polygon
A closed plane figure with at least three sides. The sides intersect only at their endpoints, and no adjacent sides are collinear.

Vertex of a Polygon
The vertex of each angle of a polygon.

Concave Polygon
A polygon for which there is a line containing a side of the polygon that also contains a point in the interior of the polygon.

Convex Polygon
A polygon for which there is NO line that contains both a side of the polygon and a point in the interior of the polygon.

ngon
A polygon with n sides.

Equilateral Polygon
A polygon whose sides are all congruent.

Equiangular Polygon
A polygon whose angles are all congruent.

Regular Polygon
A polygon that is both equiangular and equilateral.

Perimeter
The sum of the lengths of the sides of a polygon.

Circumference
The distance around a circle.

Area
The number of square units needed to cover a surface.

