Geometry is a branch of mathematics that deals with shapes and their properties - points, lines, planes and solids.

Geometry Terms

• Acute angle - An angle that measures less than 90°.
• Adjacent angle - Two angles on a plane having the same vertex and a common line.
• Angle - Two rays sharing a common point.
• Angle bisectors - A line that divides an angle into two equal angles.
• Apex - The vertex at the tip of a cone or pyramid.
• Apothem - A line segment from the center of a regular polygon to the mid point of a side.
• Arc length - The distance along the arc or the circumference of a circle or any curve.
• Area - The inside space of a figure.
• Base - The bottom of a figure in either plane or solid geometry.  If the bottom and top are parallel then either can be called base.
• Base area - The base surface area of a solid figure.
• Center of a circle - A point at a fixed equal distance from all points of the circumference of a circle.
• Central angle - An angle in a circle with the vertex at the circle's center.
• Centroid - The center of a plane or mass.
• Chord - A line segment on the interior of a circle.
• Circle segment - An interior part of a circle bound by a chord and an arc.
• Circumcircle - A circle that passes through all the vertices of a two-dimensional figure.
• Circumference - The outside of a circle or a complete circular arc.
• Concave - An internal angle is greater than 180°.
• Congruent - All sides having the same lengths. All angles measure the same.
• Conic section - A section on any plane through a circle, ellipse, hyperbola, and parabola.
• Convex - No internal angle is greater than 180°.
• Coplanar - Lines or points lying on the same plane.
• Diagonal - A line from one vertices to another that is non adjacent.
• Edge - A line where two or more vertices come together.
• Ellipsoid - A three-dimensional figure sphere like surface for which all cross sections are ellipses.
• Foci - Two fixed points on the interior of an ellipse used to define the curve.
• Focus - The points that construct a conic section.
• Geometric figure - Any set of points on a plane or in space.
• Horizontal - Flat or level.
• Hyporenuse - The side of right triangle opposite the right angle, which is the longest side.
• Inradius - A circle that touches all lines of a figure.
• Inscribed angle - An angle inside a circle with its vertex in the circle.
• Inscribed circle - The largest circle possible that can fit on the inside of a two-dimensional figure.
• Interior angle - An angle on the interior of a plane figure.
• Lateral surface area - The surface area of a solid figure excluding the area bases of the solid figure.
• Line - A straight path connecting two points.
• Line segment (segment) - All points bertween two points.
• Locus - A word used to define a set of points that form a geometric figure or graph.
• Median - A line segment from a vertex (corner point) to the midpoint of the opposite side.
• Oblique - Tilted at an angle, not horizontal or vertical.
• Oblique cone - A three-dimensional figure with the apex not alligned above the center at 90° to the base.
• Oblique cylinder - A three-dimensional figure with both bases not alligned above each other and the center not at 90° to the other base center.
• Oblique prism - A three-dimensional figure with both bases not alligned above each other and the center not at 90° to the other base center.
• Oblique pyramid - A three-dimensional figure with the apex not alligned above the center at 90° to the base.
• Obtuse angle - An angle that measures more than 90°.
• Outradius (circumcircle) - A circle that touches all corners of an figure.
• Parallel lines - Two lines that do not intersect.
• Parallel planes - Two planes that do not intersect.
• Perimeter - The sum of the lengths of all sides of a plane figure.
• Perpendicular - At a 90° angle.
• Perpendicular bisector - A line perpendicular to a segment passing through the segment's midpoint.
• Plane - An infinate expanse of points in two directions.
• Point - A single location or the intersection of two lines.
• Polyhedron - A three-dimensional figure that is a solid with no curved edges or sides.
• Porportional - Having a constant ratio.
• Pyramid - A three-dimensional figure that is a polyhedron with a polygonal base and lateral faces that taper to an apex.
• Quadrilateral - A polygon with four sides.
• Radius - A line segment between the center point and a point on a circle or sphere.
• Ray - A line starting at a point and extending infinitely in one direction.
• Rectangular angle - Two rectangles with different lengths that intersect at a 90° angle at one end each.
• Regular hexahedron - A three-dimensional figure that is a regular polyhedron where all faces are squares.
• Regular pentagon - A two-dimensional figure that is a polygon with five congruent sides.
• Regular polygon - A two-dimensional figure that is a polygon where all sides are congruent and all angles are congruent.
• Regular polyhedron - A three-dimensional figure having faces that are all congruent regular polygons and has dihedral angles that are all congruent.
• Regular prism - A three-dimensional figure having a base that are regular polygons.  The base is not necessarily aligned one above the other.
• Regular pyramid - A three-dimensional figure having a base that are regular polygons.  The apex is not necessarily above the center of the base.
• Regular right prism - A three-dimensional figure having a base that are regular polygons.
• Regular right pyramid - A three-dimensional figure having a base that are regular polygons.
• Right angle - A 90° angle.
• Right cone - A three-dimensional figure with the apex directly above the center at 90° to the base.
• Right cylinder - A three-dimensional figure with both bases directly above each other and having the centers at 90° to each others base.
• Right prism - A three-dimensional figure with both bases directly above each other and having the centers at 90° to each others base.
• Right pyramid - A three-dimensional figure with the apex directly above the center at 90° to the base.
• Secant line - A line that passes through at least two points on a curve.
• Sector of a circle - A sector is a fraction of the area of a circle with a radius on each side and an arc.
• Semi-perimeter - One half of the perimeter.
• Side - Any line segment of an figure.
• Surface - A three-dimensional figure excluding internal points.
• Surface area - The total area of all the surfaces of a three dimensional figure.
• Three-dimensional - A figure with height, width, and depth.
• Two-dimensional - A flat figure.
• Vertex - A point where two or more curves, edges, or lines meet.
• Vertical - Straight up and down.
• Vertical angles - The opposite angles when two lines intersect.
• Volume - The amount of space in an solid figure.