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Geometry vocabulary (common core)

plane

an endless flat surface

parallel lines

lines that NEVER intersect

intersecting lines

lines that pass through the same point

perpendicular lines

lines that form right angles (perfect square corners; 90 degrees)

line segment

a part of a line with two endpoints

ray

a part of a line that has one endpoint and continues on forever in one direction

right angle

measures 90 degrees; forms a perfect square corner

acute angle

measures less than a right angle

obtuse angle

measures greater than a right angle, but less than a straight angle

straight angle

forms a straight line

polygon

a closed plane figure made up of line segments

side

a line segment of a polygon

vertex

the point where two sides meet on a polygon

triangle

3-sided polygon

quadrilateral

4-sided polygon

pentagon

5-sided polygon

hexagon

6-sided polygon

octagon

8-sided polygon

equilateral triangle

has 3 equal sides

isosceles triangle

has 2 equal sides

scalene triangle

no equal sides

right triangle

has 1 right angle

acute triangle

has 3 acute angles

obtuse triangle

has 1 obtuse angle

parallelogram

has 2 pairs of parallel sides

square

*has 4 right angles
*has 2 pairs of parallel sides
*all sides are equal

rectangle

*has 4 right angles
*has 2 pairs of parallel sides
*opposite sides are equal in length

rhombus

*has 2 pairs of parallel sides
*all sides are equal

trapezoid

*has 1 pair of parallel sides

symmetric

when a figure can be folded on a line to form two equal halves that fit on top of each other

line of symmetry

the fold line

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