Geometry terms and definitions

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To save you having to refer to a dictionary, we’ve listed below some of the more common geometry terms and geometry definitions to help you help with your child’s geometry homework.

Term

Definition/ Description

 
Point A location in space - a dot on a piece of paper
Line Connects two points via the shortest path and continues indefinitely (forever) in both directions
a horizontal line through two dots with arrow symbols at each side
Line Segment Part of a line between two points
a horizontal line starting and finishing at marked points
Perpendicular Line Segment Line Segments that intersect (cross) at an angle of 90°
a horizontal line and a vertical line shonw interesecting with a right-angle symbol
Parallel Line Segments Line segments that never intersect (they are always the same distance apart)
two horizontal parallel lines
Right Angle An angle that measures 90°
two lines, AB, and BC forming a right angle
Acute Angle An angle that measures less than 90°
two lines, AB, and BC forming an acute angle
Obtuse Angle An angle that measures more than 90°
two lines, AB, and BC forming an obtuse angle
Vertex Point at which two line segments intersect (forming an angle)
two line segments with coincident endpoints forming a vertex
Scalene Triangle A triangle with all three sides with different lengths
outline of a scalene triangle
Isosceles Triangle A triangle with two equal length sides (and two equal internal angles)
outline of a isosceles triangle
Equilateral Triangle A triangle with all three sides of equal length (each internal angles = 60°)
outline of an equilateral triangle
Radius Distance (line segment) from center of a circle to any point on that circle's circumference.
outline of a circle with its center point and radius highlighted
Diameter A line segment (or length) joining two points on a circles circumference and passes through the circle's center (twice the length of the radius)
outline of a circle with its center point and diameter highlighted
Circumference Distance around a circle (the perimeter)
Chord A line segment joining two points on a curve.
outline of a circle with a chord shown in bold
Arc Part of a curve
outline of a circle with its center point an arc shown on its circumference

This Math Open Reference Project provides additional information with more geometry terms and definitions.

All Geometry 24 Worksheets Terms/ Definitions Formulas/ Equations 2D Shapes 3D Shapes Quadrilaterals Measuring Angles Using a Protractor Adding and Subtracting Angles Angle Properties Finding Angles Symmetry Area Volume Surface Area Perimeter Coordinate System Coordinate Graphing Pythagoras' Theorem Distance Between Two Points Congruent Triangles Similar Triangles Transformations Dilations

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