## Geometry

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.