  To be successful in geometry, you need to know vocabulary terms. Use this geometry glossary to help you with your studies.

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

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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 two-dimensional 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 three-dimensional 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 three-dimensional 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 six-sided 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.

Non-collinear: 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 eight-sided 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 five-sided 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 three-dimensional figure in which the two bases are congruent polygons.

Pyramid: A three-dimensional 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 a2 + b2 = c2. 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.

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.

Same-Side 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 three-dimensional 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 three-sided 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?"