## Geometry

Geometry is a branch of mathematics that deals with shapes and their properties - points, lines, planes and solids.  For other math terms see Algebra and Trigonometry.

## Geometry Terms

### A

• Acute angle - An angle that measures less than 90°.
• Adjacent angle - Two angles on a plane having the same vertex and a common line.
• Adjacent side - Next to the angle in a general triangle or next to the angle of the hypotenuse of a right triangle.
• Algorithm - A procedure used to solve a problem or a desired result.
• Altitude - The shortest distance from the base to the apex.
• Angle - Two rays sharing a common point.
• Angle bisector - A line that divides an angle into two equal angles.
• Antipodal point - Two points directly opposite each other on a sphere.
• 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.
• Axiom - A statement accepted as true without proof.

### B

• 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.
• Bisect - Divide into two congruent parts.

### C

• 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.
• Cevian - A line that extends from a vertex of a triangle to the opposite side.
• Chord - A line segment on the interior of a circle.
• Circle -  All points are at a fixed equal distance from a center point.
• Circumcenter - The point of intersection of the perpendicular bisectors of the sides.
• Circumscribed circle (circumcircle, outradius) - A circle that passes through all the vertices of a two-dimensional figure.
• Circumscribed (outside) - A sphere that touches every vertex.
• Circumscribed sphere - A polyhedron is a sphere that contains the polyhedron and touches each of the ployhedron's vertices.
• Circumference - The outside of a circle or a complete circular arc.
• Collinear - When three or more points lie on a straight line.
• Collinear points - Points that lie on the same line.
• Concave - An internal angle is greater than 180°.
• Concentric - Like geometric figures that share a common center.
• Concurrent - Lines or curves that all intersect at a single point.
• Conditional - An if... then... statement.
• Congruent - All sides having the same lengths and angles measure the same.
• Congruent lines - Line segments having the same length but not necessarily the same angle.
• Conic section - A section on any plane through a circle, ellipse, hyperbola, and parabola.
• Convex - No internal angle is greater than 180°.
• Coplanar - Lines and points lying on the same plane.
• Coplanar points - Points that lying on the same plane.

### D

• Diagonal - A line from one vertices to another that is non adjacent.
• Directrix - The path followed by a point or line when moving.

### E

• Edge - A line where two or more vertices come together.
• Ellipse - A two-dimensional figure with a conic section or a stretched circle.
• Ellipsoid - A three-dimensional figure sphere like surface for which all cross sections are ellipses.
• Euler line - The centroid, circumcenter, and orthocenter of any triangle always lies on a straight line.
• Euler's number -  $$\large{ e=2.718281828749... }$$
• Expression - Any combination of numbers and variables used to create a mathematical expression or formula.
• Exterior angle - An angle on the exterior of two rays of a plane figure.

### F

• Face - Any flat surface of a solid figure.
• Foci - Two fixed points on the interior of an ellipse used to define the curve.
• Focus - The points that construct a conic section.
• Full angle - An angle that measures 360°.

### G

• Golden ratio (divine porportion, golden mean, golden section) - A mathematical ratio commonly found in nature and design.  $$\large{ \varphi=1 : 1.6180339887... }$$
• Generatrix - A point or line when moved along a certain path creates a new shape.
• Geometric figure - Any set of points on a plane or in space.

### H

• Hexahedron - Any polyhedron with six faces.
• Horizontal - Parallel to a flat or level surface of the earth.
• Hyporenuse - The side of right triangle opposite the right angle, which is the longest side.
• Hypothesis - A tentative statement that might be true.  Identify the problem, research the problem, then make a proposed solution to the problem.

### I

• Incenter - The point where the angle bisectors meet for a regular polygon or triangle.
• Incircle - A circle that touches all lines of a figure.
• Initial side of an angle - The inside of a ray of an x-axis of an angle.
• Inscribed (inside) - A sphere that touches the center of every faces.
• 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.
• Inscribed sphere (insphere) - A convex polyhedron is a sphere that is contained within the polyhedron and tangent to each of the polyhedron's faces.
• Inside angle - An angle on the interior of two rays of a plane figure.
• Interior angle - An angle on the interior of two rays of a plane figure.
• Intersecting lines - Lines that have only point in common.
• Interval - The region or space between two defined values.

### L

• 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.
• Line segment bisector - A line or line segment that cuts another line segment into two equal parts.
• Locus - A set of points that form a geometric figure or graph.
• Long diagonal - Always crosses the center point of the figure.

### M

• Major arc - The longest of two arcs of a circle or ellipse.
• Major axis - The longest axis of a circle or ellipse.
• Mean - Another word for average.
• Median - The middle number in a data set.
• Median of a trapezoid - A line segment from the midpoint of the legs of a trapezoid.
• Median of a triangle - A line segment from a vertex (corner point) to the midpoint of the opposite side.
• Midpoint - A point on a line segment that divides the segment into two congruent segments.
• Midsphere - A polyhedron is a sphere that is tangent to every edge of the polyhedron.
• Minor arc - The shorter of two arcs of a circle or ellipse.
• Minor axis - Always the shortest axis of a circle or ellipse.
• Mixed number - A number written as a whole number and a fraction.

### N

• Non-collinear - Points that do not all lie on the same line.
• Non-collinear points - Points that do not all lie on the same line.
• Non-coplanar points - Points that do not all lie on the same plane.
• Non-polyhedron - Has a solid with curved edges or sides.
• Normal - In math it means to be at right angles.

### O

• 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°.
• Opposite angle (vertical angle) - The opposite angles when two lines intersect.
• Outlier - A data point in a data set that is far outside of an established pattern.

### P

• 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 - An angle that measures 90°.
• Perpendicular bisector - A line perpendicular to a segment passing through the segment's midpoint.
• Pi -  $$\large{ \pi=3.1415926535 ... }$$
• Plane - An infinate expanse of points in two directions.
• Plane geometry - A two dimensional figure, also called planar geometry, with edges.  The edges are line segments or edges and curve segments or arcs, all lying on a flat plane.
• Platonic solids - Each face is a regular polygon.  There are five, cube, dodecahedron, icosahedron, octahedron, and tetrahedron.
• Point - A single location or the intersection of two lines.
• Polygon - A closed plane figure for which all edges are line segments and not necessarly congruent.
• Polyhedron - A three-dimensional figure that is a solid with no curved edges or sides.
• Porportional - Having a constant ratio.
• Prism solid - A prisim is a solids with two parallel ends that are exactly the same.
• Pyramid - A three-dimensional figure that is a polyhedron with a polygonal base and lateral faces that taper to an apex.
• Pyramid solid - A polyhedrons with a polygonal base and lateral faces that taper to an apex.

### Q

• Quadrilateral - A polygon with four sides.
• Quotient - The answer after dividing one number or expression by another.

### R

• Radical - A square root.
• Radius - A line segment between the center point and a point on a circle or sphere.
• Range - The difference between the largest and smallest nunbers in a data set.
• 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.
• Reference angle - The smallest angle that the terminal side of a given angle makes with the x-axes.
• Reflection of a line section - Every point on a line segment appears an equal distance on the other side of a line.
• Right angle - An angle that measures 90°.

### S

• Sagitta - The perpendicular from the midpoint of the arc's chord to the arc itself.
• Scalene - If no two sides have the same length.
• 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.
• Segment (line segment) - All points bertween two points.
• Segment of a Circle - An interior part of a circle bound by a chord and an arc.
• Semi-major axis - Half of the longest axis of a circle or ellipse.
• Semi-minor axis - Half of the shortest axis of a circle or ellipse.
• Semicircle - Half of a circle.
• Semiperimeter - One half of the perimeter.
• Set - A defined group of objects.
• Side - Any line segment of an figure.
• Similar - Identical in shape, but not necessarily the same size.
• Skew lines - Lines that are not in the same plane and do not intersect.
• Skewed data - Not symmetric around the mean and the data (graph) has a long tail on one or the other.
• Short diagonal - Does not cross the center point of the figure.
• Solid geometry - A three-dimensional figure with connecting edges on multiple planes.  The surface of each two-dimensional plane is called a face.
• Straight angle - An angle that measures 180° or pi radians.
• Standard position of an angle - An angle where its vertex is located at the origin and one ray is on the positive x-axis.
• Supplement of an angle - Any angle A between 0° and 180°.  A is 180° - A
• Supplementary angles - Two angles that sum to 180°.
• Surface - A three-dimensional figure excluding internal points.
• Surface area - The total area of all the surfaces of a three dimensional figure.
• Symmetric - When two or more parts are congruent or identical to each other.

### T

• Terminal side of an angle - The inside of a ray of an angle oposite the x-axis initial side.
• Three-dimensional - A figure with height, width, and depth.
• Translation - Moving a shape to a new location with no other changes.
• Transversal - A line that intersects two other lines.
• Triangle exterior angle theorm - An exteroir angle is equal to the sum of the opposite interior angles.
• Triangle inequality theorem - Any side of a triangle is always shorter than the sum of the other two sides.
• Triangle inequality theorem converse - A triangle cannot be constructed from three line segments if any of them is longer than the sum of the other two.
• Two-dimensional - A flat figure.

### V

• Variable - A symbol used to represent a number that we do not know yet.
• Vector - A quantity with both magnitude and direction.
• Vertex - A point where two or more curves, edges, or lines meet.
• Vertices - Plural of vertex.
• Vertical - Perpendicular to horizontal or straight up or down.
• Vertical angle (opposite angle) - The opposite angles when two lines intersect.
• Volume - The amount of space in an solid figure.
Display #
Title
Acute Angle
Angle
Angle Types
Apex
Apothem
Area
Arm of an Angle
Base
Chord of a Circle
Congruence of Segments
Congruent Angles
Congruent Complementary
Congruent Right Angle
Congruent Supplements
Congruent Vertical Angles
Diagonal
Edge
Euclid's Postulates
Exterior Angles
Face
Geometry Terms
Heron's Formula
Interior Angles
Leg
Line Segment