Definitions: 2D shapes
This page will provide definitions for the vocabulary we will be using during the twodimensional geometry unit. Definitions come from various sources, of which, the easiest definition was chosen for students to understand.
polygon: a closed figure with three or more straight sides. We name a polygon by the number of its sides. (ie. a fivesided polygon is a pentagon)
Number of sides
on the polygon Name of the Polygon
*3 
triangle 
*4 
square, rectangle
rhombus, trapezoid

*5 
pentagon 
*6 
hexagon 
7 
heptagon 
*8 
octagon 
9 
nonagon 
10 
decagon 
11 
hendecagon 
12 
dodecagon 
13 
tridecagon 
20 
icosagon 
*grade three students only need to know the * names
Angle: two sides of a figure, or lines, meet to form an angle
Right angle: two sides of a figure, or lines, that make a square corner at the vertex form a right angle.
Obtuse angle: two sides of a figure, or lines that make an angle larger than a right angle.
Acute angle: two sides of a figure, or lines that make an angle smaller than a right angle.
Congruent: figures (shapes) that have the same size and same shape.
Figure: a geometic drawing or diagram
Parallel lines: two lines that are always the same distance apart and never meet
Parallelogram: a figure with 4 sides where 2 pairs of opposite sides are parallel
*we use arrows to show which pairs of lines
are parallel
Regular figure: has all sides equal and all angles are equal. A square is a regular figure.
Rhombus: a parallelogram with 4 equal sides.
Reflection (Flip): reflects a figure in a line to create a congruent image.
Rotation (Turn): a turn moves a figure around a turn centre. After 1 turn, the figure is back to where it started.
Slide (Translation): a slide moves a figure along a line. The figure does not turn.
Trapezoid: a figure with 4 sides where 2 sides are parallel.
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