Geometry Basic Concepts: Point, Line, Plane, and Angle

A set of vocabulary flashcards covering the fundamental concepts of Grade 9 geometry including undefined terms, subsets of lines, and angle components.

Point

The basic geometric term that indicates a specific location in space; it has no size, no width, no length, and no depth, and is commonly represented by a small dot.

Line

A straight path that extends infinitely in two opposite directions; it is one-dimensional, possessing only length.

Plane

A flat surface that extends infinitely in all directions along its surface; it is two-dimensional, possessing length and width but no thickness.

Undefined Terms

The basic ideas of point, line, and plane that are used to describe and define all other geometric figures, serving as the building blocks of geometry.

Line Segment

A part of a line consisting of two endpoints and all the points in between; unlike a line, its length can be measured.

Ray

A part of a line that starts at one endpoint and extends infinitely in one direction.

Angle

A geometric figure formed by two rays that share a common endpoint.

Vertex

The common endpoint shared by the two rays that form an angle.

Sides

The two rays that form an angle.

Collinear Points

Points that lie on the same line.

Non-collinear points

Points that do not lie on the same line; three such points are required to determine a plane.

Point Notation

The method of naming a point using a single CAPITAL letter, such as Point $A$.

Line Notation

The method of naming a line using a single lowercase italic/script letter (e.g., line $m$) or by any two points on it with a double-arrow bar above them (e.g., $\overleftrightarrow{AB}$).

Plane Notation

The method of naming a plane using a single capital script letter (e.g., Plane $P$) or by naming any three non-collinear points on the plane (e.g., Plane $ABC$).

Line Segment Notation

Written using the letters of its two endpoints with a plain bar over them, such as $\overline{AB}$ or $\overline{BA}$.

Ray Notation

Written with a one-directional arrow over two letters, where the first letter must be the endpoint, such as $\overrightarrow{AB}$.