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| A line is a straight path that extends without end in opposite directions. A line is named by two points on the line. |
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| A point is an exact location. A point is named by a capital letter. |
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| A plane is a flat surface that extends without end in all directions. A plane is named by three points on the plane that are not on the same line. |
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| A line segment is made of two endpoints and all the points between the endpoints. A line segment is named by its endpoints. |
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| A ray has one endpoint. From the endpoint, the ray extends without end in one direction only. A ray is named by its endpoint first followed by another point on the ray. |
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| An angle is formed by two rays with a common endpoint. |
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| The vertex is a common endpoint for an angle formed by two rays. |
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| An acute angle measures less than 90°. |
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| A right angle measures exactly 90°. |
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| An obtuse angle measures more than 90° and less than 180°. |
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| A straight angle measures exactly 180°. |
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| When angles have the same measure, they are said to be congruent. |
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| Vertical angles are formed opposite each other when two lines intersect. Vertical angles have the same measure, so they are always congruent. |
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| Adjacent angles are side by side and have a common vertex and ray. Adjacent angles may or may not be congruent. |
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| Complementary angles are two angles whose measures have a sum of 90°. |
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| Supplementary angles are two angles whose measures have a sum of 180°. |
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| Parallel lines are lines in the same plane that never intersect. |
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| Perpendicular lines intersect to form 90° angles, or right angles. |
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| Skew lines are lines that lie in different planes. They are neither parallel nor intersecting. |
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| An acute triangle has only acute angles. |
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| An obtuse triangle has one obtuse angle. |
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| A right triangle has one right angle. |
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| A scalene triangle has no congruent sides. |
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| An isosceles triangle has at least two congruent sides. |
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| An equilateral triangle has three congruent sides. |
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| A quadrilateral is a plane figure with four sides and four angles. |
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| Opposite sides are parallel and congruent. Opposite angles are congruent. |
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| Parallelogram with four right angles. |
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| Parallelogram with four congruent sides. |
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| Rectangle with four congruent sides. |
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| Quadrilateral with exactly two parallel sides. May have two right angles. |
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| A polygon is a closed plane figure formed by three or more line segments. |
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| A regular polygon is a polygon in which all sides are congruent and all angles are congruent. |
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| A rigid transformation moves a figure without changing its size or shape. So the original figure and the transformed figure are always congruent. |
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| A translation is the movement of a figure along a straight line. |
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| A rotation is the movement of a figure around a point. A point of rotation can be on or outside a figure. |
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| When a figure flips over a line, creating a mirror image, it is called a reflection. |
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| When a figure flips over a line, creating a mirror image, it is called a reflection. The line the figure is flipped over is called line of reflection. |
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Definition
| A figure has line symmetry if it can be folded or reflected so that the two parts of the figure match, or are congruent. |
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Definition
| If a figure has line symmetry, the line of reflection is called the line of symmetry. |
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