Geometry for Elementary School/Glossary

Below is a glossary of words used in this book.

A

Acute angle

An angle that is less than 90 degrees

Adjacent angle

When two angles share the same vertice, and one same but opposite arm

Adjacent angles on a straight line

When angles add up to 180 degrees and are adjacent

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

When angles add up to 360 degrees and are adjacent

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

Quadrilateral

A polygon with four sides

R

Radius

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