Download Acute Angle An angle that has a measure of less then 90 degrees

Glossary of Geometry Terms
Acute Angle An angle that has a measure of less then 90
degrees.
Angle A figure formed by two rays (sides) with the same endpoint (vertex). A right angle has
a measure of 90 degrees. An acute angle has a measure of less than 90 degrees. An obtuse
angle has a measure greater than 90 degrees but less than 180 degrees. A straight angle
has a measure of 180 degrees. Example: Hands of a clock
Base a. A side of a polygon, usually the one at the bottom in a given position.
b. A face of a three-dimensional figure, usually the one on which it stands in a given
position.
Chord A line segment that connects two points on a circle.
Circle A closed, curved two-dimensional figure having all points an equal distance from the
center.
Closed Figure A two-dimensional figure that starts and ends at the same point.
Cone A three-dimensional figure with one curved surface, one vertex, and one circular base.
Congruent Figures Figures that have the same shape and size.
Corresponding Parts Matching parts of congruent figures.
Cube A prism, all of whose faces are squares.
Cylinder A three-dimensional figure with one curved surface and two circular bases.
Decagon A polygon with ten sides and ten angles.
Diagonal A line segment other than a side that connects two vertices of a polygon.
Diameter A chord that passes through the center of a circle.
Edge A line segment where two faces of a three-dimensional figure meet.
Endpoint A point at the end of a ray or a line segment.
Equilateral Triangle A triangle with three congruent sides.
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Exterior (of an angle) All points that are not on the ray on in the interior of an angle.
Face A flat side of a three-dimensional figure.
Flip To reflect a figure over a line; reflection.
Hexagon A polygon with six sides and six angles.
Initial Ray Of two rays that form an angle, the one that stays fixed in place while the terminal
ray swings out from it.
Interior (of an angle) All the points between the rays of an angle.
Intersecting Lines Lines that meet or cross at a common point.
O
Irregular Polygon A polygon whose sides and angles are not all of the same lengths and
measures.
Isosceles Triangle A triangle with two congruent sides.
Kite A quadrilateral with two pairs of adjacent congruent sides.
Line A straight path that goes in two directions without end. Example: Straight railroad
tracks
Line of Symmetry A line that divides a figure into two parts that match, or are congruent.
Line Segment A part of a line between two endpoints. Example: Edge of a book
Obtuse Angle An angle that has a measure greater than 90 degrees but less than 180
degrees.
Obtuse Triangle A triangle with one obtuse angle.
Octagon A polygon with eight sides and eight angles.
Open Figure A two dimensional figure that does not start and end at the same point.
Parallel Lines Lines in the same plane that never intersect.
Parallelogram A quadrilateral with opposite sides that are parallel. Each pair of opposite
sides and angles are congruent.
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Geometry Glossary of Terms
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Pentagon A polygon with five sides and five angles.
Perpendicular Lines Intersecting lines that cross each other at right angles.
Plane A flat surface that is endless in all directions. Example: Floor or wall of a room
Plane Figure Another name for two-dimensional figure.
Polygon A closed two-dimensional figure with sides that are line segments. The sides do not
cross each other.
Polygon
triangle
quadrilateral
pentagon
hexagon
octagon
decagon
Number of Sides and Vertices
3
4
5
6
8
10
Prism A three-dimensional figure with two parallel congruent bases. The rest of the faces
are rectangles or parallelograms.
Protractor An instrument used to draw or measure angles.
Pyramid A three-dimensional figure that is shaped by triangles on a base.
Quadrilateral A four-sided polygon.
Radius A line segment that connects a point on a circle with the center.
Ray A part of a line that has one endpoint and continues in one direction without end.
Example: Beam of a flashlight
Rectangle A parallelogram with four right angles.
Rectangular Prism A prism, all of whose faces are rectangles.
Rectangular Pyramid A pyramid whose base is a rectangle.
Regular Polygon A polygon with congruent sides and congruent angles.
Rhombus A parallelogram with four congruent sides.
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Geometry Glossary of Terms
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Right Angle An angle that has a measure of exactly 90 degrees.
Right Triangle A triangle with one right angle.
Scalene Triangle A triangle with no congruent sides.
Similar Figures Figures that have the same shape but not necessarily the same size.
Skew Lines Any lines in space that do not intersect and are not parallel.
Slide To move a figure along a line; translation.
Sphere A three-dimensional figure that is the set of all points that are a given distance from
a fixed point.
Square A rectangle with four congruent sides.
Square Pyramid A pyramid whose base is a square.
Square Pyramid A pyramid whose base is a square.
Straight Angle An angle that has a measure of exactly 180 degrees.
Symmetric A figure is symmetric if it can be folded along a line so that the resulting halves
match exactly, or are congruent.
Terminal Ray Of two rays that form an angle, the one that opens out from the initial ray.
Tessellation An arrangement of repeating geometric figures that cover an area without any
overlaps or gaps.
Three-Dimensional Figure A figure that has length, width, and height.
Trapezoid A quadrilateral with exactly one pair of parallel lines.
Triangle A three-sided polygon.
Triangular Prism A prism with two parallel, congruent bases that are triangles.
Triangular Pyramid A pyramid whose base is a triangle.
Turn The movement of a figure by rotating it around a point; rotation.
Two-Dimensional Figure A figure that has only length and width. Examples: angles,
polygons, circles
Vertex The common point of the two rays of an angle, two sides of a polygon, or three or
more edges of a three-dimensional figure. Example: Corner of a page
Baltimore County Public Schools
Department of Professional Development
Parent Support Services, 4/2005
Geometry Glossary of Terms
PSS Resources/Curriculum Resources/Elementary/Math