Cone

Cone of Revolution

Congruence of Geometric Figures

Congruent

Congruent Angles

Congruent Figures

Congruent Line Segments

Congruent Polygons

Congruent Sides

Congruent Triangles

Conic Section

Conical Perspective

Conical Surface

Conjugate Axis

Constant

Contraction

Convex

Convex Polygon

Convex Polyhedron

Convex Quadrilateral

Convex Solid

Convexity

Coordinate

Coordinate System

Coordinates of a Point

Coplanar

Coplanar Lines

Coplanar Points

Corresponding

Corresponding Angles (Lines Cut by a Transversal)

Corresponding Angles (Similar Figures)

Corresponding Elements

Corresponding Sides

Cosine of an Angle

Cube

Curvature

Curve

Curved line

Cylinder

Cylinder of Revolution

Cylindrical Surface

Decagon

Decahedron

Decomposable Solid

Degree of an Angle

Deltahedron

Deltoid

Diagonal

Diagonal of a Polygon

Diagonal of a Polyhedron

Diameter

Dihedral Angle

Dilation in a Cartesian Plane

Dimension

Dimension of a Vector Space

Dimetric Perspective

Dimetric Projection

Direct Isometry

Direct Variation Function

Directed Line Segment

Direction of Rotation

Directrix

Disphenoid

Distance

Distance Between a Point and a Line

Distance Between Two Parallel Lines

Distance Between Two Points on a Cartesian Plane

Distance Between Two Points on a Line

Dodecagon

Dodecahedron

Eccentricity of a Conic Section

Economic Function

Edge

Edge of a Geometric Solid

Edge of a Polygonal Chain

Ellipse

Ellipse in a Cartesian Plane

Enlargement

Enneagon

Equality

Equiangular

Equiangular Polygon

Equidistant

Equilateral

Equilateral Hyperbola

Equilateral Polygon

Equilateral Triangle

Equipollence

Equipollent Bipoints

Equivalent

Equivalent Figures

Equivalent Solids

Eratosthenes (C. 276 BCE – C.194 BCE)

Error

Escribed Circle, or Excircle, of a Triangle

Euclid (~300 B.C.)

Euler’s Formula

Exterior Angle

Exterior Angle of a Circle

Exterior Angle of a Polygon