
Please click the link below to jump to the terms starting with that letter. Otherwise, feel free to scroll through the entire glossary! 




Acute Angle: Angles that measure between 0 and 90 degrees. 

Acute Triangle: A triangle where all 3 angles are less than 90 degrees. More about triangles. 

Adjacent Angles: Angles that are next to each other. 

Alternative Exterior Angles: Angles on opposite sides of the transversal that are on the exterior of the two parallel lines. More. 

Alternate Interior Angles: Angles on opposite sides of the transversal that are also between the two parallel lines. More. 

Altitude: The height of a triangle. 

Angle: Two rays (lines or segments) that have a common endpoint. More information. 

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. 

Angle of Depression: An angle formed by a horizontal line of sight and an object viewed below it. 

Angle of Elevation: An angle formed by a horizontal line of sight and an object viewed above it. 

Area: The amout of square units that cover a twodimensional surface. 

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. 



Base Angle (Isosceles Triangle): The two (congruent) angles of an isosceles triangle. They are form by each leg and the base side. 

Centroid: The point of a triangle that is created by the intersection of its three medians. Sometimes referred to as the "center of gravity." 

Circle: The set of all points in a given plane that is equidistant from a given point, known as the center of the circle. 

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. 

Circumference: The distance around a circle. 

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). 

Collinear: Points that lie on the same line. 

Complementary Angles: Two angles whose sum add to exactly 90 degrees. 

Concave Polygon: A polygon with at least one pair of adjacent sides that are bent inward. 

Concentric Circles: Multiple circles in a plane in which they have different diameters, but share the same center. 

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. 

Cone: A threedimensional figure that has a circle for its base and connects at a given point known as the vertex. 

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. 

Congruent Angles: Angles that have the same degree measure. 

Congruent Polygons: Polygons that have the exact same size and shape. 

Congruent Segments: Segments that have the same length. 

Converse: A reversed conditional statement. The "if" and "then" parts of the conditional statement are switched. 

Convex Polygons: A polygon in which none of the sides are bent inward (like a concave polygon). 

Coplanar: on the same plane. 

Corresponding Angles: Two angles in the same "spot" when parallel lines are intersected by a transversal. More. 

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. 

Cylinder: A threedimensional figure with 2 circular bases. 



Diameter: The distance from one point on a circle to another point on the circle which runs through the center. 

Distance: The distance between 2 points. The distance must be in a "straight line." 

Equilateral Triangle: A triangle in which all three sides are congruent. 

Exterior Angles: Angles that are on the outside of the two parallel lines that are intersected by a transversal. More. 

Hexagon: A sixsided polygon. 

Hypotenuse: The longest side of a right triangle. The hypotenuse is always across from the right angle. 



Incenter: The point of a triangle that is created by the intersection of its three angle bisectors. 

Inscribed Polygon: A polygon in which all of its vertices intersect on a circle. The polygon is said to be "inscribed in the circle." 

Interior Angles: Angles between the two parallel lines that are intersected by the transversal. More. 

Intersecting Lines: Two lines that cross at exactly one ponit. 

Isosceles: A figure with two congruent sides. The most common examples are isosceles triangle and isosceles trapezoid. 



Kite: A quadrilateral with exactly two pairs of adjacents that are congruent. 

Line: A set of points that extend forever in both directions. 

Linear Pair: Two adjacent angles that form a line. These angles are always supplementary. 

Median: A segment inside a triangle that intersects a vertice and the midpoint of an opposite side. 

Midpoint: The point that divides a segment into two equal parts. It is the middle point of the segment. 



Noncollinear: Points that are not all on the same line. 

Noncoplanar: Figures that are not all within in the same plane. 

Obtuse Angle: An angle with a measure between 90 and 180 degrees. 

Obtuse Triangle: A triangle which contains an obtuse angle. More about triangles. 

Octagon: An eightsided polygon. 

Orthocenter: The point of a triangle that is created by the intersection of its three altitudes. 



Parallel Lines: Two lines in a plane that do not intersect. More. 

Parallelogram: A quadrilateral in which both pairs of opposite sides are parallel. 

Pentagon: A fivesided polygon. 

Perimeter: The distance around a polygon. 

Perpendicular Bisector: A segment (ray, line) that intersects a segment at a right angle through its midpoint. 

Perpendicular Lines: Lines that intersect to form right angles. 

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. 

Pi: The ratio of a circles circumference to its diameter. More about pi. 

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. 

Polygon: A closed plane figure. 

Prism: A threedimensional figure in which the two bases are congruent polygons. 

Pyramid: A threedimensional figure in which the base is a polygon and the sides converge to a given point known as the vertex. 

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 a^{2} + b^{2} = c^{2}. Learn how to use the pythagorean theorem, or learn about Pythagoras, the man for which the theorem is named. 

Pythagorean Triples: Three numbers which can be the sides of a right triangle because they fit the pythagorean theorem. 



Quadrilateral: A foursided polygon 

Radius: The distance from a center to a point on a circle. 

Ray: A set of points that extends forever in exactly one direction. More information. 

Rectangle: A quadrilateral with four congruent angles (all are 90 degrees). 

Regular Polygon: A polygon in which all angles are congruent, and all sides are also congruent. 

Rhombus: A quadrilateral in which all four sides are congruent. 

Right Angle: An angle whose measure is 90 degrees. 

Right Triangle: A triangle which posesses a right angle. More about triangles. 



SameSide Interior Angles: Two angles that are between the two parallel lines and are also on the same side of the transversal. More. 

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. 

Scalene Triangle: A triangle in which all three sides are not congruent (different lengths). More about triangles. 

Segment: A set of points with distinct endpoints that does not extend forever in either direction. It is basically a piece of a line. 

Segment Bisector: A segment (line, ray) that intersects another segment through its midpoint. 

Similar Polygons: Polygons that are the same shape, but not necessarily the same size. 

Skew Lines: Two lines that do not intersect, but are not in the same plane. 

Sphere: The set of all points in space that are equidistant from a given point, knowns as the center. 

Square: A quadrilateral in which all sides are congruent, and all angles are also congruent. 

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. 

Supplementary: Two angles whose sum is 180 degrees. 

Surface Area: The sum of all the areas of a threedimensional figure. 



Transformation: A change in size, shape, or position of a geometric figure. 

Transversal: A line that intersects two or more other lines (often those lines are parallel). More. 

Trapezoid: A quadrilateral in which exactly one pair of opposite sides are parallel. 

Triangle: A threesided polygon. More about triangles. 

Vertex: The common endpoint of two rays. 

Vertical Angles: Angles that are across from each other. Vertical angles are formed by intersecting lines, and their measures are always equal. 

Vertices: Plural form of the word vertex. 

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?" 


