# Geometry for Elementary School/Glossary

 Geometry for Elementary School What next? Glossary Conventions

Below is a glossary of words used in this book.

## A

Acute angle
An angle that is less than 90 degrees
When two angles share the same vertice, and one same but opposite arm
Adjacent angles on a straight line
Altitude
The line drew from the base to the tip of the triangle
Angle sum of triangles
The angle sum of triangles is 180 degrees
Angles at a point
Arc
A part of a circumference
Area
How big a plane shape is
Arms
One of the two rays in an angle
Axiom
Similar to a theorem, except it is thought up rather than proved

## B

Base
Any side of the triangle

## C

Centre
The middle of a shape, especially a circle
Chord
A line that goes from one point of the circumference to another with touching the centre
Circle
A shape with infinite loci that are the same distance from the circle
Colinear
When two points are on the same line
Complementary angles
Angles that add up to 90 degrees
Cone
A shape with a circle as a base and a pointy top
Coplanar
When two points or lines are on the same plane.
Cross-section
The shape formed after cutting a solid
Cube
A hexahedron with all edges equal
Cuboid
A rectangular prism
Curved surface
A surface that is not smooth
Cylinder
A shape with parallel, congruent circles as bases

## D

Decagon
A polygon with ten sides
Decahedron
A polyhedron with ten sides
Degree
The unit used for measuring angles
Diameter
The line that goes from one point of the circumference, through the centre and to another side of the circumference
Dimension
The kind of space that a shape is in. If a shape has 2-dimensions, it has width and length; if it has three, it has width, height and length.
Dissecting
A method of finding the area of a polygon
Dodecagon
A polygon with twelve sides
Dodecahedron
A polyhedron with twelve sides

## E

Edge
The edge of a solid
Ellipse
A shape that looks like an oval
Equiangular
When a polygon's angles are identical
Equilateral
When a polygon's sides are identical
Euler's formula
Faces + Vertices - Edges = 2

## F

Face
A plane figure on a solid
Filling
A method of finding the area of a polygon
Finite
Not unlimited
Focus
One of the two points in the middle of a ellipse

## H

Height
The third dimension of a solid or the altitude of a triangle
Heptagon
A polygon with seven sides
Heptahedron
A polyhedron with seven sides
Hexagon
A polygon with six sides
Hexahedron
A polyhedron with six sides

## I

Intersect
When two or more lines, planes, etc., cross each other
Irregular
Not regular
Isosceles trapezium
A trapezium with two sides equal
Isosceles triangle
A triangle with two sides identical

## K

Kite
A quadrilateral where two pairs of adjacent sides are equal and one pair of opposite angles is equal

## L

Line
A series of colinear points which extends in both directions forever; has no width
Line of intersection
The line formed when two planes intersect
Line segment
A line with a fixed length, with two endpoints

## N

Nonagon
A polygon with nine sides
Nonahedron
A polyhedron with nine sides

## O

Obtuse angle
An angle between 90 and 180 degrees
Octagon
A polygon with eight sides
Octahedron
A polyhedron with eight sides

## P

Parallel line
Two lines that never intersect
Parallelogarm
A quadrilateral where opposite sides and angles are equal
Pentagon
A polygon with five sides
Pentahedron
A polyhedron with five sides
Perimeter
The sum of a plane shape's sidse
Perpendicular line
When two lines intersect to create right angles
Pi
The ratio of the diameter and the circumference
Plane
A plane that extends in all directions forever; has no thickness
Point
A dot that is so small that its height and width are actually zero
Point of intersection
The point formed when two lines intersect
Polygon
A shape that has straight sides
Postulate
See Axiom
Prism
A shape with parallel, congruent bases
Protractor
A tool for measuring angles
Pyramid
A shape with a base and a pointy top

## Q

A polygon with four sides

## R

The line that goes from the centre to one point of the circumference
Ray
A line that only extends in one direction forever
Rectangle
A quadrilateral opposite sides are equal and all angles are 90 degrees
Reflex angle
An angle between 180 and 360 degrees
Regular
When a polygon is both equilateral and equiangular, or when a solid satisfies all the criteria for regular polyhedra
Rhombus
A quadrilateral where all sides are equal and opposite angles are equal
Right angle
An angle of 90 degrees
Right-angled trapezium
A trapezium with a right angle
Round angle
An angle of 360 degrees

## S

Scalene trapezium
A trapezium with neither a right angle nor equal sides
Semicircle
Half a circle
Septagon
See Heptagon
Septahedron
See Heptahedron
Sphere
A ball
Splitting
See Dissecting
Square
A regular quadrilateral where all sides are equal and all angles are equal
Straight angle
An angle of 180 degrees
Supplementary angles
Angles that add up to 180 degrees

## T

Tetrahedron
A polyhedron with four sides
Total surface area
The total area of all faces of a solid
Trapezium
A quadrilateral where there is one pair of parallel sides
Trapezoid
See Trapezium
Triangle
A polygon with three sides

## U

Undecagon
A polygon with eleven sides
Undecahedron
A polyhedron with eleven sides
Uniform cross-sections
The identical shapes formed after cutting the solid in the same direction; only on prisms and cylinders

## V

Vertex
A corner in things such as angles, plane figures and solids
Vertically opposite angles
When angles share the same vertex and their opposite arms are on the same straight line; are always the same size.
Volume
How big a solid is