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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  
Line Segment  Part of a line between two points  
Perpendicular Line Segment  Line Segments that intersect (cross) at an angle of 90°  
Parallel Line Segments  Line segments that never intersect (they are always the same distance apart)  
Right Angle  An angle that measures 90°  
Acute Angle  An angle that measures less than 90°  
Obtuse Angle  An angle that measures more than 90°  
Vertex  Point at which two line segments intersect (forming an angle)  
Scalene Triangle  A triangle with all three sides with different lengths  
Isosceles Triangle  A triangle with two equal length sides (and two equal internal angles)  
Equilateral Triangle  A triangle with all three sides of equal length (each internal angles = 60°)  
Radius  Distance (line segment) from center of a circle to any point on that circle's circumference.  
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)  
Circumference  Distance around a circle (the perimeter)  
Chord  A line segment joining two points on a curve.  
Arc  Part of a curve 
This Math Open Reference Project provides additional information with more geometry terms and definitions.
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