Geometry terms

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Line
Continues forever in both directions
Line segment
Ends in both directions
Ray
Continues forever in one direction
Obtuse angle
An angle greater than 90 degrees
Right angle
An angle exactly 90 degrees
Acute angle
An angle less than 90 degrees
Polygon
A shape that has at least 3 sides, is closed, has no line segments that cross, and has only straight line segments, like a triangle or quadrilateral
Circle
A perfectly round 2D shape
Triangle
A 3-sided polygon
Quadrilateral
A 4-sided polygon
Pentagon
A 5-sided polygon
Hexagon
A 6-sided polygon
Heptagon
A 7-sided polygon
Octagon
An 8-sided polygon
Nonagon
A 9-sided polygon
Decagon
A 10-sided polygon
Acute triangle
A triangle with 3 acute angles
Obtuse triangle
A triangle with 1 obtuse angle
Right triangle
A triangle with 1 right angle
Parallelogram
A quadrilateral with 2 pairs of parallel sides
Trapezoid
A quadrilateral with 1 pair of parallel sides
Rectangle
A parallelogram with 4 right angles
Rhombus
A parallelogram with 4 equal sides
Square
A rectangle with 4 equal sides; it is a rhombus, a rectangle, a parallelogram, and a quadrilateral
Kite
A quadrilateral with 2 sides the same length right next to each other, and 2 other sides the same length right next to each other
Equilateral triangle
A triangle with 3 equal sides
Isosceles triangle
A triangle with 2 equal sides
Scalene triangle
A triangle with all 3 sides different lengths
Polyhedron
A 3D shape made of polygons
Prism
A polyhedron with sides that are rectangles
Cube
A prism that has all sides squares
Cylinder
A 3D shape with 2 bases that are circles
Sphere
A 3D shape like a 3D circle
Pyramid
A polyhedron with all sides triangles
Face
The flat side of a 3D shape
Vertex/vertices
Where 3 or more faces meet, like a corner
Edge
Where 2 faces meet
Volume
The space inside a 3D shape, the length x width x height
Area
The space inside a 2D shape, the length x width, like the area of a carpet
Perimeter
The length around a 2D shape, like a fence around something
Symmetry
When it looks the same on both sides, or would look identical if you turned it
Similar
The same shape, but can be different sizes
Congruent
The exact same shape and size, but can be turned
Parallel
Two lines that will never cross
Perpendicular
Two lines that cross to form right angles
Intersect
Two lines that cross, could be perpendicular, could be any other angle as long as they will cross