Term | Definition |
Line | A line has no thickness and it extends forever in two directions. |
Line segment | A subset of a line that contains two points of the line and all points between those two points. |
Ray | A subset of the line AB that contains the
endpoint A, the point B, all points between A and B, and all points C on the line such that B is between A and C. |
Skew line | Lines GF and DE are skew lines. They do not
intersect, and there is no plane that contains them. |
Concurrent line | Lines DE, EG, and EF are concurrent
lines; they intersect at point E. |
Parallel line | Line m is parallel to line n. They have no points in common. |
Axiom | Axioms are statements that cannot be proven
and are assumed. |
Theorem | Theorems are statements that can be proven
using axioms and logic. |
Half plane | Line AB separates plane "a" into two half-planes. |
Angle | An angle is formed by two rays with the same endpoint. |
Vertex | Vertex is the common endpoint of the two rays that form an angle. |
Sides of an angle | The sides of an angle are the two rays that form an angle. |
Adjacent angle | Adjacent angles are two angles with a common
vertex and a common side, but without
overlapping interiors. |
Radian | An angle of 1 radian is an angle whose vertex is at the center of a circle and that intercepts an arc equal in length to the radius of the circle.
1 radian ≈ 57.296 degrees |
A line perpendicular to a plane | A line perpendicular to a plane is a line that is perpendicular to every line in the plane through its intersection with the plane. |
Dihedral angle | A dihedral angle is formed by the union of two half-planes and the common line defining
the half-planes. |
Dimensional analysis (or unit analysis) | Dimensional analysis (or unit analysis) is a process to convert from one unit of measurement to another. |
Triangle inequality theorem | The sum of the lengths of any two sides of a triangle is greater than the length of the third side.
AB + BC > AC |
Perimeter | Perimeter is the length of a simple closed curve, or the sum of the lengths of the sides of a polygon. |
Circle | Circle is the set of all points in a plane that are the same distance from a given point, the center. |
Circumference | Circumference is the perimeter of a circle. |
Pi | Pi is the ratio between the circumference of a circle and the length of its diameter. |
Simple curve | Simple curve is a curve that does not cross itself; starting and stopping points may be the same. |
Closed curve | Closed curve is a curve that starts and stops at the same point. |
Polygon | A polygon is a simple, closed curve with sides that are line segments. |
Convex curve | Convex curve is a simple, closed curve with no indentations; the segment connecting any two points in the interior of the curve is wholly contained in the interior of the curve. |
Concave curve | Concave curve is a simple, closed curve that is not convex; it has an indentation. |
Interior angle | Interior angle is an angle formed by two sides of a polygon with a common vertex. |
Diagonal | Diagonal is a line segment connecting
nonconsecutive vertices of a polygon. |
Exterior angle of a convex polygon | Exterior angle of a convex polygon is an angle formed by a side of a polygon and the extension of a contiguous side of the polygon. |
Congruent part | Congruent parts are parts with the same size and shape. |
Regular polygon | If all sides of a polygon are congruent and all angles are congruent, the polygon is a
regular polygon. A regular polygon is equilateral and equiangular. |
Right triangle | Right triangle is a triangle containing a
right angle. |
Acute triangle | Acute triangle is a triangle in which all the
angles are acute. |
Obtuse triangle | Obtuse triangle is a triangle containing an
obtuse angle. |
Scale triangle | Scalene triangle is a triangle with no congruent sides. |
Isoceles triangle | Isosceles triangle is a triangle with at least two congruent sides. |
Equilateral triangle | Equilateral triangle is a triangle with three
congruent sides. |
Trapezoid | Trapezoid is a quadrilateral with at least
one pair of parallel sides. |
Kite | Kite is a quadrilateral with two adjacent sides congruent and the other two sides also congruent. |
Isoceles trapezoid | Isoceles trapezoid is a trapezoid with congruent base angles. |
Parallelogram | Parallelogram is a quadrilateral in which
each pair of opposite sides is parallel. |
Rectangle | Rectangle is a parallelogram with a right angle. |
Rhombus | Rhombus is a parallelogram with two adjacent sides congruent. |
Square | Square is a rectangle with two adjacent sides congruent. |
Line of symmetry | Mathematically, a geometric figure has a line of symmetry ℓ if it is its own image under a reflection in ℓ. |
Rotational (turn) symmetry | A figure has rotational symmetry, or turn
symmetry, when the traced figure can be rotated less than 360° about some point, the
turn center, so that it matches the original figure. |
Point symmetry | Any figure that has rotational symmetry 180° is said to have point symmetry about the turn center. |
Vertical angle | Vertical angles are created by intersecting lines are a pair of angles whose sides are two pairs of opposite rays. |
Supplementary angle | The sum of the measures of two supplementary
angles is 180°. |
Complementary angle | The sum of the measures of two complementary
angles is 90°. |
Transversal and angle | Angles formed when a line (a transversal)
intersects two distinct lines. |
Angle and parallel line property | If any two distinct coplanar lines are cut by a transversal, then a pair of corresponding
angles, alternate interior angles, or alternate exterior angles are congruent if, and only if, the lines are parallel. |
The sum of the measures of the interior angles of a triangle | Since the angles appear to form a straight angle, we conjecture that the angles sum up to 180 degrees. |
The sum of the measures of the exterior angles of a convex n - gon | The sum of the measures of the exterior angles of a convex n - gon is 360 degrees. |
Similar object | Similar objects have the same shape but not
necessarily the same size. |
Congruent object | Congruent objects have the same shape and the same size. |
Arc | An arc of a circle is any part of the circle that can be drawn without lifting a pencil. |
Center of an arc | The center of an arc is the center of the circle containing the arc. |
Semicircle | If the two arcs determined by a pair of points on the circle are the same size, each is a semicircle. |
Chord | A segment connecting two points on a circle is a chord of the circle. |
Diameter | If a chord contains the center, it is a diameter. |
Triangle congruence | Two figures are congruent if it is possible to fit one figure onto the other so that matching parts coincide. |
Side, side, side congruence condition (SSS) | If the three sides of one triangle are congruent, respectively, to the three sides of a second triangle, then the triangles are congruent. |
Side, angle, side property (SAS) | If two sides and the included angle of one triangle are congruent to two sides and the included angle of another triangle, respectively, then the two triangles are congruent. |
Hypotenuse leg theorem | If the hypotenuse and a leg of one right triangle are congruent to the hypotenuse and a leg of another right triangle, then the triangles are congruent. |
Altitude | An altitude of a triangle is the perpendicular segment from a vertex of the triangle to the line containing the opposite side of the triangle. |
Angle, Side, Angle (ASA) property | If two angles and the included side of one
triangle are congruent to two angles and the
included side of another triangle, respectively, then the triangles are congruent. |
Angle, angle, side (AAS) | If two angles and a side opposite one of these two angles of a triangle are congruent to the two corresponding angles and the corresponding side in another triangle, then the two triangles are congruent. |
Properties of angle bisectors | Any point P on an angle bisector is equidistant from the sides of the angle. same. Any point that is equidistant from the sides of an angle is on the angle bisector of the angle. |
Incenter of a triangle | The angle bisectors of a triangle are concurrent (they intersect in a single point, the incenter) and the three distances from the point of intersection to the sides are equal. |
Similar polygon | Two polygons with the same number of vertices are similar if there is a one-to-one correspondence between the vertices of one and the vertices. |
AA similarity for triangle | If two angles of one triangle are congruent,
respectively, to two angles of a second triangle, then the triangles are similar. |
SSS similarity for triangle | If corresponding sides of two triangles are
proportional, then the triangles are similar. |
SAS similarity for triangle | Given two triangles, if two sides are proportional and the included angles are
congruent, then the triangles are similar. |
Properties of proportion | If a line parallel to one side of a triangle intersects the other sides, then it divides those sides into proportional segments. |
Properties of proportion | If parallel lines cut off congruent segments on one transversal, then they cut off congruent segments on any transversal. |
Midsegment of triangles and quadrilaterals | The midsegment (segment connecting the
midpoints of two sides of a triangle) is parallel to the third side of the triangle and half as long. |
Median | A median of a triangle is a segment connecting a vertex of the triangle to the midpoint of the opposite side. |
Center of gravity or centroid | The three medians are concurrent. The point of intersection, G, is the center of gravity, or the centroid, of the triangle. |
Indirect measurement | Similar triangles have long been used to make
indirect measurements. |