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Geometry Dictionary

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Translation
A slide
Rotation
A turn
Reflection
A flip
A plane
Flat surface extending forever in all directions
A line
Straight path that extends forever in both directions
Line segment
Straight path between two endpoints
A angle
A figure formed by two rays that begin at the same point
Vertex
End point
A point
Exact location in space
Polygon
Closed figure formed by line segments joined only a their endpoints
Parallel lines
Lines that do not cross
Perpendicular lines
Lines that cross at 90 degree angle
Hexagon
Polygon with six sides
Octagon
Polygon with eight sides
Decagon
Polygon with ten sides
Right angle
Angle that is 90 degrees
Regular polygon
All sides congruent and all angles congruent
Diagonal
A line segment that joins two vertices of a polygon but it not a side
Obtuse angle
Greater than 90 degrees and less than 180 degrees
Acute angle
Angle less than 90 degrees
Nonagon
Polygon with nine sides
Heptagon
Polygon with seven sides
Pentagon
Polygon with five sides
Equilateral triangle
Three sides that have the same length
Isosceles triangle
Two sides that have the same length
Scalene triangle
No sides that have the same length
Acute triangle
Less than 90 degrees
Right triangle
90 degrees angle
Obtuse triangle
Greater than 90 degrees
Trapezoid
One pair of opposite sides parallel
Parallelogram
Opposite sides parallel, opposite sides congruent
Rhombus
Parallelogram, 4 congruent sides
Rectangle
Parallelogram, 4 right angles
Square
Parallelogram, 4 right angles, 4 congruent sides
Transformation
The movement of a geometric figure
Intersecting Lines
A straight path that has one end point and extends forever in the opposite direction
Congruent figures
Figures that are the same size and same shape
Similar figures
Figures that are the same shape and may or may not have the same size
Line of symmetry
Place where a figure can be folded so that both halves are congruent
Complementary angles
Angles that add up to 90
Supplementary angles
Angles that add up to 180