Definitions:  2D shapes

This page will provide definitions for the vocabulary we will be using during the two-dimensional 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 five-sided polygon is a pentagon)

Number of sides

on the polygon                 Name of the Polygon

*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.