# 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