GLOSSARY OF GEOMETRY TERMS
Acute Angle An angle with a measure greater than 0 degrees but less than 90 degrees.
Acute Triangle A triangle with three acute angles (or no angle measuring 90 degrees or greater).
Adjacent Sides Two sides of a polygon with a common vertex.
Altitude An altitude of a triangle is a line segment from a vertex of the triangle perpendicular to the line containing the side opposite that vertex.
Angle Two rays that share an endpoint.
Arc Part of a circle.
Area The amount of surface in a region or enclosed within a boundary.
Attribute A characteristic possessed by an object. Characteristics include shape, color, size, length, weight, capacity, area, etc.
Base of a Solid A plane figure. If the solid is a cylinder or prism, there are two bases that are parallel and congruent.
Centimeter A metric unit of length equal to one-hundredth of one meter.
Circle A closed curve with all points in one plane and equidistant from a fixed point (the center).
Circumference The length of the boundary of a circular region.
Concentric Circles Two or more circles that have the same center and different radii.
Concave Polygon A concave polygon is any polygon with an angle measuring more than180°. Concave polygons look like they are collapsed or have one or more angles dented in.
Cone A three-dimensional figure with a circular base joined to a vertex by a curved surface.
Convex polygon A convex polygon is any polygon that is not concave.
Congruent Having exactly the same size and shape. Congruent polygons have their corresponding angles congruent and corresponding sides congruent.
Coordinate System A reference system for locating and graphing points. In two dimensions, a coordinate system usually consists of a horizontal axis and a vertical axis, which intersect at the origin. Each point in the plane is located by its horizontal distance and vertical distance from the origin. These distances, or coordinates, form an ordered pair of numbers.
Cube A solid figure in which every face is a square and every edge is the same length.
Cubic Foot The volume of a cube that is one foot wide, one foot high, and one foot long.
Cubic Unit A unit of measure that has a length of one unit, a width of one unit, and a height of one unit used to measure volume. Examples are cubic inches, cubic centimeters, etc.
Cylinder A solid figure formed by two congruent parallel circles joined by a curved surface.
Decagon A polygon with ten sides. A regular decagon has ten congruent sides and ten congruent angles.
Diagonal A line segment that joins two non-adjacent vertices of a polygon or polyhedron.
Diameter A line segment passing through the center of a circle or sphere and connecting two points on the circle or sphere.
Dimension The number of coordinates used to express a position.
Dodecagon A polygon with twelve sides. A regular dodecagon has twelve congruent sides and twelve congruent angles.
Dodecahedron A polyhedron with twelve faces. All faces of a regular dodecahedron are congruent, regular pentagons.
Edge A line segment where two faces of a three-dimensional figure meet.
Endpoint The point(s) at the end of a ray or line segment.
Equilateral Refers to a figure in which all sides have the same length.
Equiangular Refers to a figure in which all angles have the same measure.
Equilateral Triangle A triangle with three congruent sides.
Face A plane figure that serves as one side of a solid figure.
Flip (See Reflection)
Geometry The branch of mathematics that deals with the position, size, and shape of figures.
Grid A network of horizontal and vertical lines that intersect to form squares or rectangles.
Hemisphere Half of a sphere, formed by making a plane cut through the center of a sphere.
Heptagon A polygon with seven sides. A regular heptagon has seven congruent sides and seven congruent angles.
Hexagon A polygon with six sides. A regular hexagon has six congruent sides and six congruent angles.
Hexahedron A polyhedron with six faces. A regular hexahedron is a cube.
Hypotenuse The side opposite the right angle of a right triangle. The hypotenuse is the longest side of a right triangle.
Icosahedron A polyhedron with twenty faces. All faces of a regular icosahedron are congruent, equilateral triangles.
Isosceles Triangle A triangle with at least two congruent sides. (An equilateral triangle is a special case of an isosceles triangle.)
Kite A convex quadrilateral with two distinct pairs of adjacent, congruent sides.
Line A set of points that form a straight path extending infinitely in two directions. Part of a line with two endpoints is called a “line segment”.
Line of Symmetry A line dividing a two-dimensional figure into two parts that are mirror images of each other.
Line Segment A part of a line. A line segment has two endpoints and a finite length.
Median A median of a triangle is a segment connecting any vertex of a triangle to the midpoint of the opposite side.
Net A net is a two-dimensional representation of a three-dimensional object.
Nonagon A polygon with nine sides. A regular nonagon has nine congruent sides and nine congruent angles.
Obtuse Angle An angle that is greater than 90 degrees but less than 180 degrees.
Obtuse Triangle A triangle that has one obtuse angle.
Octagon A polygon with eight sides. A regular octagon has eight congruent sides and eight congruent angles.
Octahedron A polyhedron with eight faces. All faces of a regular octahedron are congruent, equilateral triangles.
Opposite Angles In a quadrilateral, angles that do not have a common side--that is, non-adjacent angles.
Parallel Lines Lines lying in the same plane that are always the same distance apart.
Parallelogram A quadrilateral with both pairs of opposite sides parallel.
Pentagon A polygon with five sides. A regular pentagon has five congruent sides and five congruent angles.
Perimeter The distance around a figure.
Perpendicular At right angles.
pi (p) The ratio of the circumference of a circle to its diameter. This ratio is the same for every circle. Its value, which is found by dividing the circumference by the diameter, is a little more than 3.14.
Pie graph A circle marked into sectors. Each sector shows the fraction represented by one category of data. Pie graphs are also called circle graphs.
Plane A flat surface extending infinitely in all directions.
Plane Figure In geometry, a closed two-dimensional figure that lies entirely in one plane. (Polygons and circles are examples of plane figures. An arc is not a plane figure because it is not closed.)
Point The smallest geometric unit. A position in space, often represented by a dot.
Polygon A 2-D figure that is simple, closed, and composed of line segments.
Polyhedron A solid figure bounded by polygons.
Prism A polyhedron with at least one pair of opposite faces that are parallel and congruent. Corresponding edges of these faces are joined by rectangles or parallelograms. Prisms are named by the base polygons (e.g., triangular prism, square prism, hexagonal prism, etc).
Pyramid A polyhedron with any polygon for its base. The other faces are triangles that meet at a point or vertex called the apex of the pyramid. Pyramids are named by the base polygon (e.g., triangular pyramid, square pyramid, pentagonal pyramid, etc). Most familiar are pyramids in which the apex is directly above the center of the base; these are called right pyramids. Pyramids that are not right are called oblique.
Quadrilateral A polygon with four sides.
Ray A set of points that form a straight path extending infinitely in one direction. A ray has one endpoint.
Rectangle A parallelogram with four right angles.
(Right) Rectangular A solid figure in which all six faces are rectangles with three pairs of parallel, congruent, opposite faces.
Reflection A transformation of a geometric figure that results in a mirror image of the original.
Regular Polygon A polygon that has all congruent sides and all congruent angles—i.e., a polygon that is both equilateral and equiangular.
Regular Polyhedron A polyhedron with congruent regular polygons as faces and has congruent angles between all faces.
Reflex Angle An angle with measure greater than 180o but less than 360o.
Rhombus A parallelogram with four congruent sides.
Right Angle An angle that is one-fourth of a full turn. A right angle measures 90 degrees.
Right Triangle A triangle that has one right angle.
Scalene Triangle A triangle with no sides congruent.
Semicircle One-half of a circle, also called a semi-circle.
Similar Figures that have the same shape but not necessarily the same size. Similar polygons have corresponding angles congruent and corresponding sides in proportion. Congruent is a special case of similar where the ratio of the corresponding sides is 1-1.
Sides The line segments that form a polygon.
Slide (See Translation)
Solid Figure A closed, three-dimensional figure.
Sphere A three-dimensional figure formed by a set of points that are all the same distance from a fixed point called the center.
Square A rectangle with congruent sides.
Square Unit A unit of measure that has a length of one unit and a width of one unit used to measure area. Examples are square inches, square centimeters, acres, etc.
Surface Part or all of the boundary of a solid. A surface may be flat or curved. (For example, a cone has one flat surface and one curved surface).
Symmetry a. If a figure can be folded along a line so that the two halves match exactly, then the figure has line symmetry.
b. If a figure can be turned less than 360 degrees about a point and fit exactly on itself, then a figure has turn (or rotational) symmetry.
Tessellation An arrangement of plane figures (usually congruent figures) to cover a surface without overlapping or leaving any gaps.
Tetrahedron A polyhedron with four triangular faces. A tetrahedron is a triangular pyramid.
Three-Dimensional Relating to objects that have length, width, and depth. Solid figures such as polyhedra, cones, and spheres are three-dimensional.
Transformation Moving a geometric figure from one position to another, according to a rule. Examples of transformations are reflection, rotation, and translation.
Translation A transformation in which a geometric figure is formed by moving every point on a figure the same distance in the same direction.
Trapezoid A quadrilateral with exactly one pair of parallel sides.
Triangle A polygon with three sides.
Triangular Prism A prism in which the bases are triangles.
Two-Dimensional Relating to figures that have length and width but not depth. Figures such as polygons and circles are two-dimensional.
Vertex a. A point at which two line segments, lines, or rays meet to form an angle.
b. A point on a polyhedron where three or more faces intersect.
The plural of vertex is vertices.