To be successful in geometry, you need to know vocabulary terms. Use this geometry glossary to help you with your studies.
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| Please click the link below to jump to the terms starting with that letter. Otherwise, feel free to scroll through the entire glossary! |
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| Acute Angle: Angles that measure between 0 and 90 degrees. |
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| Acute Triangle: A triangle where all 3 angles are less than 90 degrees. More about triangles. |
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| Adjacent Angles: Angles that are next to each other. |
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| Alternative Exterior Angles: Angles on opposite sides of the transversal that are on the exterior of the two parallel lines. More. |
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| Alternate Interior Angles: Angles on opposite sides of the transversal that are also between the two parallel lines. More. |
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| Altitude: The height of a triangle. |
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| Angle: Two rays (lines or segments) that have a common endpoint. More information. |
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| Angle Bisector: A ray (line or segment) that intersects the vertex of an angle in such a way that it creates two congruent (equal) angles. |
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| Angle of Depression: An angle formed by a horizontal line of sight and an object viewed below it. |
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| Angle of Elevation: An angle formed by a horizontal line of sight and an object viewed above it. |
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| Area: The amout of square units that cover a two-dimensional surface. |
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| ASA (Angle Side Angle Triangle Congruence): A "shortcut" for proving two triangles are congruent by proving the two pairs of angles and the pair of included sides are congruent. |
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| Base Angle (Isosceles Triangle): The two (congruent) angles of an isosceles triangle. They are form by each leg and the base side. |
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| Centroid: The point of a triangle that is created by the intersection of its three medians. Sometimes referred to as the "center of gravity." |
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| Circle: The set of all points in a given plane that is equidistant from a given point, known as the center of the circle. |
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| Circumcenter: The point of a triangle that is created by the intersection of its three perpendicular bisectors. This point is useful because it is equidistant to the three vertices of the triangle. |
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| Circumference: The distance around a circle. |
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| Circumscribed: A polygon is said to be circumscribed about a circle if each side of the polygon intersects the circle in exactly one point (known as a point of tangency). |
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| Collinear: Points that lie on the same line. |
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| Complementary Angles: Two angles whose sum add to exactly 90 degrees. |
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| Concave Polygon: A polygon with at least one pair of adjacent sides that are bent inward. |
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| Concentric Circles: Multiple circles in a plane in which they have different diameters, but share the same center. |
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Conditional: A statement made up of two clauses known as the "hypothesis" and the "conclusion." The words if and then are used before each of those terms, respectively.
Example: If it rains outside, then I will use my umbrella. |
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| Cone: A three-dimensional figure that has a circle for its base and connects at a given point known as the vertex. |
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| Congruent: having the same size and shape. Geometric figures are said to be "congruent" while numbers are said to be "equal." They are very similar, but a little different. |
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| Congruent Angles: Angles that have the same degree measure. |
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| Congruent Polygons: Polygons that have the exact same size and shape. |
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| Congruent Segments: Segments that have the same length. |
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| Converse: A reversed conditional statement. The "if" and "then" parts of the conditional statement are switched. |
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| Convex Polygons: A polygon in which none of the sides are bent inward (like a concave polygon). |
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| Coplanar: on the same plane. |
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| Corresponding Angles: Two angles in the same "spot" when parallel lines are intersected by a transversal. More. |
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| CPCTC (Corresponding Parts of Congruent Triangles are Congruent): When two triangles are proven to be congruent, then each of their corresponding parts (sides and angles) are also congruent. |
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| Cylinder: A three-dimensional figure with 2 circular bases. |
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| Diameter: The distance from one point on a circle to another point on the circle which runs through the center. |
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| Distance: The distance between 2 points. The distance must be in a "straight line." |
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| Equilateral Triangle: A triangle in which all three sides are congruent. |
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| Exterior Angles: Angles that are on the outside of the two parallel lines that are intersected by a transversal. More. |
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| Hexagon: A six-sided polygon. |
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| Hypotenuse: The longest side of a right triangle. The hypotenuse is always across from the right angle. |
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| Incenter: The point of a triangle that is created by the intersection of its three angle bisectors. |
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| Inscribed Polygon: A polygon in which all of its vertices intersect on a circle. The polygon is said to be "inscribed in the circle." |
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| Interior Angles: Angles between the two parallel lines that are intersected by the transversal. More. |
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| Intersecting Lines: Two lines that cross at exactly one ponit. |
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| Isosceles: A figure with two congruent sides. The most common examples are isosceles triangle and isosceles trapezoid. |
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| Kite: A quadrilateral with exactly two pairs of adjacents that are congruent. |
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| Line: A set of points that extend forever in both directions. |
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| Linear Pair: Two adjacent angles that form a line. These angles are always supplementary. |
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| Median: A segment inside a triangle that intersects a vertice and the midpoint of an opposite side. |
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| Midpoint: The point that divides a segment into two equal parts. It is the middle point of the segment. |
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| Non-collinear: Points that are not all on the same line. |
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| Noncoplanar: Figures that are not all within in the same plane. |
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| Obtuse Angle: An angle with a measure between 90 and 180 degrees. |
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| Obtuse Triangle: A triangle which contains an obtuse angle. More about triangles. |
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| Octagon: An eight-sided polygon. |
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| Orthocenter: The point of a triangle that is created by the intersection of its three altitudes. |
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| Parallel Lines: Two lines in a plane that do not intersect. More. |
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| Parallelogram: A quadrilateral in which both pairs of opposite sides are parallel. |
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| Pentagon: A five-sided polygon. |
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| Perimeter: The distance around a polygon. |
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| Perpendicular Bisector: A segment (ray, line) that intersects a segment at a right angle through its midpoint. |
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| Perpendicular Lines: Lines that intersect to form right angles. |
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| Plane: A set of points that forms a flat surface and extends forever in all directions. It is one of the three undefined terms in geometry. |
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| Pi: The ratio of a circles circumference to its diameter. More about pi. |
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| Point: The most basic geometric figure. It is represented by a dot but it has no size or shape. It is one of the three undefined terms in geometry. |
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| Polygon: A closed plane figure. |
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| Prism: A three-dimensional figure in which the two bases are congruent polygons. |
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| Pyramid: A three-dimensional figure in which the base is a polygon and the sides converge to a given point known as the vertex. |
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| Pythagorean Theorem: For a right triangle, the sum of the square of both legs equals the square of the hypotenuse. It is commonly written with the formula a2 + b2 = c2. Learn how to use the pythagorean theorem, or learn about Pythagoras, the man for which the theorem is named. |
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| Pythagorean Triples: Three numbers which can be the sides of a right triangle because they fit the pythagorean theorem. |
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| Quadrilateral: A four-sided polygon |
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| Radius: The distance from a center to a point on a circle. |
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| Ray: A set of points that extends forever in exactly one direction. More information. |
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| Rectangle: A quadrilateral with four congruent angles (all are 90 degrees). |
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| Regular Polygon: A polygon in which all angles are congruent, and all sides are also congruent. |
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| Rhombus: A quadrilateral in which all four sides are congruent. |
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| Right Angle: An angle whose measure is 90 degrees. |
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| Right Triangle: A triangle which posesses a right angle. More about triangles. |
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| Same-Side Interior Angles: Two angles that are between the two parallel lines and are also on the same side of the transversal. More. |
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| SAS (Side Angle Side Triangle Congruece): A "shortcut" for proving two triangles are congruent by proving that two pairs of sides and the pair of included angles are congruent. |
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| Scalene Triangle: A triangle in which all three sides are not congruent (different lengths). More about triangles. |
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| Segment: A set of points with distinct endpoints that does not extend forever in either direction. It is basically a piece of a line. |
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| Segment Bisector: A segment (line, ray) that intersects another segment through its midpoint. |
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| Similar Polygons: Polygons that are the same shape, but not necessarily the same size. |
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| Skew Lines: Two lines that do not intersect, but are not in the same plane. |
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| Sphere: The set of all points in space that are equidistant from a given point, knowns as the center. |
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| Square: A quadrilateral in which all sides are congruent, and all angles are also congruent. |
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| SSS (Side Side Side Triangle Congruence): A "shortcut" for proving two triangles are congruent by proving that all corresponding sides of one triangle are congruent to all corresponding sides of the other triangle. |
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| Supplementary: Two angles whose sum is 180 degrees. |
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| Surface Area: The sum of all the areas of a three-dimensional figure. |
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| Transformation: A change in size, shape, or position of a geometric figure. |
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| Transversal: A line that intersects two or more other lines (often those lines are parallel). More. |
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| Trapezoid: A quadrilateral in which exactly one pair of opposite sides are parallel. |
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| Triangle: A three-sided polygon. More about triangles. |
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| Vertex: The common endpoint of two rays. |
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| Vertical Angles: Angles that are across from each other. Vertical angles are formed by intersecting lines, and their measures are always equal. |
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| Vertices: Plural form of the word vertex. |
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| Volume: The number of cubic units which can fill a given space. A good way to think about this is by asking yourself, "how much water can I pour into that figure?" |
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