Geometry terms and definitions

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.

TermDefinition/ Description 
PointA location in space – a dot on a piece of paper
LineConnects two points via the shortest path and
continues indefinitely (forever) in both directions
Line SegmentPart of a line between two points
Perpendicular Line SegmentLine Segments that intersect (cross) at an angle of 90°
Parallel Line SegmentsLine segments that never intersect
(they are always the same distance apart)
Right AngleAn angle that measures 90°
Acute AngleAn angle that measures less than 90°
Obtuse AngleAn angle that measures more than 90°
VertexPoint at which two line segments intersect
(forming an angle)
Scalene TriangleA triangle with all three sides with different lengths
Isosceles TriangleA triangle with two equal length sides
(and two equal internal angles)
Equilateral TriangleA triangle with all three sides of equal length
(each internal angles = 60°)
RadiusDistance (line segment) from center of a circle
to any point on that circle’s circumference.
DiameterA line segment (or length) joining two points on
a circles circumference and passes through the
circle’s center (twice the length of the radius)
CircumferenceDistance around a circle (the perimeter)
ChordA line segment joining two points on a curve.
ArcPart of a curve

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