acute angle : An angle that measures less than 90°.
acute triangle: A triangle with all angles measuring less than 90°.
adjacent angles: Angles in the same plane that have a common vertex and a common side.
angle: A figure formed by two rays with a common endpoint called the vertex.
arc: A part of a circle named by its endpoints.
asymmetry: Not identical on either side of a central line; not symmetrical.
center (of a circle): The point inside a circle that is the same distance from all the points on the circle.
center (of rotation): The point about which a figure is rotated.
central angle of a circle: An angle with its vertex at the center of a circle.
chord: A line segment with endpoints on a circle.
circle: The set of all points in a plane that are the same distance from a given point called the center.
complementary angles: Two angles whose measures add to 90°.
congruent: Having the same size and shape.
corresponding angles (for lines): A pair of angles formed by a transversal and two lines.
corresponding angles (in polygons): Matching angles of two or more polygons.
diameter: A line segment that passes through the center of a circle and has endpoints on the circle, or the length
of that segment.
equilateral triangle: A triangle with three congruent sides.
image A figure resulting from a transformation.
isosceles triangle: A triangle with at least two congruent sides.
line: A straight path that extends without end in opposite directions.
line of reflection: A line that a figure is flipped across to create a mirror image of the original figure.
line of symmetry: The imaginary “mirror” in line symmetry.
line segment: A part of a line between two endpoints.
line symmetry: A figure has line symmetry if one-half is a mirror image of the other half.
obtuse angle: An angle whose measure is greater than 90° but less than 180°.
obtuse triangle: A triangle containing one obtuse angle.
parallel lines: Lines in a plane that do not intersect.
parallelogram: A quadrilateral with two pairs of parallel sides.
perpendicular lines: Lines that intersect to form right angles.
plane: A flat surface that extends forever.
point: An exact location in space.
polygon: A closed plane figure formed by three or more line segments that intersect only at their endpoints
(vertices).
radius: A line segment with one endpoint at the center of a circle and the other endpoint on the circle, or the
length of that segment.
ray: A part of a line that starts at one endpoint and extends forever.
rectangle: A parallelogram with four right angles.
reflection: A transformation of a figure that flips the figure across a line.
regular polygon: A polygon with congruent sides and angles.
rhombus: A parallelogram with all sides congruent.
right angle: An angle that measures 90°.
right triangle: A triangle containing a right angle.
rotation: A transformation in which a figure is turned around a point.
rotational symmetry: A figure has rotational symmetry if it can be rotated less than 360° around a central point
and coincide with the original figure.
scalene triangle: A triangle with no congruent sides.
sector A region enclosed by two radii and the arc joining their endpoints.
Side-Side-Side (SSS): A rule stating that if three sides of one triangle are congruent to three sides of another
triangle, then the triangles are congruent.
skew lines: Lines that lie in different planes that are neither parallel nor intersecting.
square (geometry): A rectangle with four congruent sides.
straight angle: An angle that measures 180°.
supplementary angles: Two angles whose measures have a sum of 180°.
transformation: A change in the position or orientation of a figure.
translation: A movement (slide) of a figure along a straight line.
transversal: A line that intersects two or more lines.
trapezoid: A quadrilateral with exactly one pair of parallel sides.
vertex: On an angle or polygon, the point where two sides intersect.
vertical angles: A pair of opposite congruent angles formed by intersecting lines.