# Geometry for Elementary School/Glossary

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Below is a glossary of words used in this book.

## A[edit]

- 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[edit]

- Base
- Any side of the triangle

## C[edit]

- 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[edit]

- 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[edit]

- 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[edit]

- 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

## G[edit]

## H[edit]

- 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[edit]

- 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

## J[edit]

## K[edit]

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

## L[edit]

- 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

## M[edit]

## N[edit]

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

## O[edit]

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

## P[edit]

- 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[edit]

- Quadrilateral
- A polygon with four sides

## R[edit]

- 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[edit]

- 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[edit]

- 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[edit]

- 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[edit]

- 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