Welcome to Geometry Unit 1 Vocabulary

Undefined Terms Point In Euclidean geometry, a point is undefined. You can think of a point as a location. A point has no size. You can represent a point by a dot and name it by a capital letter, such as A. Example P

Undefined Terms Line In Euclidean geometry, a line is undefined. You can think of a line as a straight path that extends in two opposite directions without end and has no thickness. A line contains infinitely many points. In spherical geometry, you can think of a line as a great circle or a sphere. You can name a line by any two points on the line, such as “line AB” or “line BA,” or by a single lower case letter, such as “line L.” A line has a measure of 180 degrees.

Undefined Terms Plane In Euclidean geometry, a plane is undefined. You can think of a plane as a flat surface that extends without end and has no thickness. A plane contains infinitely many lines. You can name a plane by at least three points in the plane that do not all lie on the same line, such as “plane ABC,” or a capital letter such as “plane P.” Example

Collinear points Collinear points lie on the same line.

Coplanar Coplanar figures are figures in the same plane.

Space Space is the set of all points. One-dimension Two-dimension Three-dimension N-dimension

Line Segment A segment is the part of a line that consists of two points, called endpoints, and all points between them. You can name a segment by its two endpoints, such as “segment DE” or “segment ED.”

Ray A ray is the part of a line that consists of one endpoint and all the points of the line on one side of the endpoint. You can name a ray by its endpoint and another point on the ray, such as “ray AB”. The order of points indicates the ray’s direction. You cannot name this as “ray BA.”

Opposite rays Opposite rays are collinear rays with the same endpoint. They form a line.

Postulate A postulate, or axiom, is an accepted statement of fact.

Intersection The intersection of two or more geometric figures is the set of points the figures have in common.

Coordinate The coordinate of a point is its distance and direction from the origin of a number line. The coordinates of a point on a coordinate plane are in the form (x,y), where x is the x-coordinate and y is the y-coordinate.

Congruent segments Congruent segments are segments that have the same length.

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

Segment bisector A segment bisector is a line, segment, ray, or plane that intersects a segment at its midpoint.

Angle An angle is formed by two rays with the same endpoint. The rays are the sides of the angle and the common endpoint is the vertex of the angle

Vertex The endpoint of the angle is the vertex of the angle.

Acute angle An acute angle is an angle whose measure is between 0 and 90.

Right angle A right angle is an angle whose measure is 90.

Obtuse angle An obtuse angle is an angle whose measure is between 90 and 180.

Straight angle A straight angle is an angle whose measure is 180.

Congruent angles Congruent angles are angles that have the same measure.

Adjacent angle Adjacent angles are two coplanar angles that have a common side and a common vertex but no common interior points.

Vertical angles Vertical angles are two angles whose sides form two pairs of opposite rays.

Complementary angles Two angles are complementary angles if the sum of their measures is 90.

Supplementary angles Two angles are supplementary if the sum of their measures is 180.

Linear pair A linear pair is a pair of adjacent angles whose non-common sides are opposite rays. Angles that are a linear pair are supplementary angles.

Angle bisector An angle bisector is a ray that divides an angle into two congruent angles.