Acute Angle: |
An acute angle is an angle that measures less than 90 degrees. |

Altitude: |
An altitude of a triangle is a line segment from one of its vertices which meets the opposite side at a right angle. |

Angle: |
An angle is a measure of the amount of rotation about the point of intersection of two lines or line segments that is required to bring one into correspondence with the other. |

Area: |
Area is a measure of the amount of material that would be needed to "cover" a surface completely. |

Circle: |
A circle is the set of points in a plane that are equidistant from a given center point. |

Circumference: |
The circumference of a circle is the length of its perimeter. |

Collinear: |
Three or more points are said to be collinear if they lie on the same straight line. |

Complementary Angles: |
Complementary angles are a pair of angles whose measures add up to 90 degrees. |

Diameter: |
(1) In plane geometry, the diameter is the straight-line distance between two points opposite one another across the center of a circle. (2) In solid geometry, the diameter is the straight-line distance between two antipodal points on a sphere. |

Geometric Construction: |
A geometric construction is a construction of a geometric figure using only straightedge and compass, as originally studied by the ancient Greeks. |

Golden Ratio: |
The golden ratio φ is a mathematical constant obtained as the ratio of longest-to-shorted side lengths in a rectangle constructed so that when it is partitioned into a square and new rectangle, the new rectangle has the same ratio of side lengths as the original. The golden ratio has value of approximately 1.618. |

Golden Rectangle: |
A golden rectangle is a rectangle in which the ratio of the longest to shortest sides is equal to the golden ratio, give approximately by 1.618. Such rectangles appear particularly promnentl in art and architecture. |

Hypotenuse: |
The hypotenuse is the longest side of of a right triangle, which is always opposite the right angle. |

Midpoint: |
The midpoint is the point on a line segment that divides it into two segments of equal length. |

Obtuse Angle: |
An obtuse angle is an angle that measures greater than 90 degrees and less than 180 degrees. |

Parallel: |
In two-dimensional Euclidean space, two lines that do not intersect are said to be parallel. In three-dimensional Euclidean space, parallel lines not only fail to intersect, but also maintain a constant separation between points closest to each other on the two lines. |

Perimeter: |
The perimeter is a length around the boundary of a closed two-dimensional region. The perimeter of a circle is called its circumference. |

Perpendicular: |
Two lines, vectors, planes, etc. that intersect at a right angle are said to be perpendicular. |

Pi: |
Pi is the mathematical constant defined as the ratio of the circumference of a circle to its diameter with value of approximately 3.14159. |

Plane Geometry: |
Plane geometry is the portion of geometry dealing with figures in a plane, as opposed to solid geometry. |

Point: |
A point is a zero-dimensional mathematical object that can be specified in *n*-dimensional space using *n* coordinates. |

Radius: |
The radius of a circle is the distance from its center to its circumference or from the center of a sphere to its surface. The radius is equal to half the diameter. |

Supplementary Angles: |
Supplementary angles are pairs of angles whose measures sum to 180 degrees. |

Triangle Inequality: |
The triangle inequality states that the sum of the lengths of any two sides of a triangle must be greater than the length of the third side. |