Acute angle – an angle measuring less than 90 degrees

Acute triangle – a triangle with exactly 3 acute angles

Adjacent angles – angles that share a common vertex and a common side and have nonoverlapping interiors

Alternate exterior angles – a pair of exterior angles that are on different sides of the transversal and are not supplementary

Alternate interior angles – a pair of interior angles that are on different sides of the transversal and are not supplementary

Altitude of a triangle – a perpendicular segment which connects a vertex of the triangle to the line containing the opposite side from the vertex

Angle – formed by two rays which share a common endpoint

Apex – the point of a pyramid

Arc – any part of a circle that can be drawn without lifting your pencil

Area – the number of square units required to cover a region with no gaps or overlaps

B

Base – the upper and lower parallel faces of a prism

Bisect – to divide into two congruent pieces

C

Center of an arc – the center of a circle containing the arc

Circle – the set of all points in a plane that lie the same distance from a given point (the center)

Circular cone/cylinder – a cone/cylinder whose base(s) are circular regions

Circumference – the perimeter of a circle

Collinear – points that lie in the same line

Complementary angles – a pair of angles whose measures sum to 90 degrees

Concave polygons – a polygon in which it is possible to draw a segment between two points of the polygonal region such that part of the segment lies outside of the region (it has “caves”)

Concurrent lines – lines that contain the same point; may be coplanar or noncoplanar

Cone – a simple-closed surface with one base which is a simple-closed curve (other than a polygon) which is connected by line segments to a point off the curve

Congruent – two objects are congruent if they have the same size and the same shape

Convex polygon – a polygon in which it is impossible to draw a segment between two points of the polygonal region such that part of the segment lies outside of the region (it does not have “caves”)

Coplanar – points (or lines) that lie in the same plane

Corresponding angles – a pair of angles that are in the same “position” relative to the two figures to which they belong

Cube – a regular polyhedron with 6 square-shaped sides

Cylinder – a simple-closed surface with two congruent, parallel bases which are simple-closed curves (other than a polygon)

D

Decagon – a polygon with 10 sides

Diagonal – a segment drawn within a polygon whose endpoints are non-consecutive vertices of the polygon

Dihedral angle – the union of two half-planes; ex. The union of the wall and the floor of our classroom

Dodecagon – a polygon with 12 sides

Dodecahedron – a regular polyhedron with 12 pentagon-shaped sides

E

Equilateral triangle – a triangle in which all three sides have the same measure

Exterior angle – an angle which does not lie between two parallel lines and is formed by one of the lines and a transversal

Exterior angle of a polygon – an angle formed by a side of the polygon and the extension of a contiguous side of the polygon

F

Faces – the polygonal regions of a polyhedron

Flip – see reflection

G

Glide reflection – a motion which combines a translation and a reflection

H

Half-turn – a rotation of 180 degrees

Heptagon – a polygon with 7 sides

Hexagon – a polygon with 6 sides

Hypotenuse – in a right triangle, the leg opposite the right angle

I

Icosahedron – a regular polyhedron with 20 triangle-shaped sides

Interior angle – an angle which lies between two parallel lines and is formed by one of the lines and a transversal

Interior angle of a polygon – an angle formed within a polygon by two sides of the polygon sharing a common vertex

Intersecting lines – two coplanar lines that have exactly one point in common

Isometry – any motion that preserves distance; also called rigid motion

Isosceles trapezoid – a trapezoid in which the two non-parallel sides are congruent

Isosceles triangle – a triangle in which at least two sides have the same length

J

K

Kite – a quadrilateral with at least two distinct pairs of consecutive sides congruent

L

Lateral faces – the faces other than bases of a prism

Line of symmetry – a figure has a line of symmetry if it is its own image under a reflection in the line; if you fold the figure along the line of symmetry it falls on top of itself

M

Major arc – an arc which is larger than a semicircle

Minor arc – an arc which is smaller than a semicircle

N

Net – a two-dimensional pattern that can be used to construct a three-dimensional figure

Nonagon – a polygon with 9 sides

O

Oblique cylinder/cone – a “leaning” cylinder

Oblique prism – a prism which has faces that are not all bounded by rectangles; a prism which “leans”

Obtuse angle – an angle which measures greater than 90 degrees

Obtuse triangle – a triangle with exactly one obtuse angle

Octagon – a polygon with 8 sides

Octahedron – a regular polyhedron with 8 triangle-shaped sides

P

Parallel lines – two distinct coplanar lines that have no points in common

Parallelogram – a quadrilateral in which each pair of opposite sides is parallel

Pentagon – a polygon with 5 sides

Perimeter – the distance around a figure

Perpendicular bisector – a line which is perpendicular to a segment and it bisects the segment

Perpendicular lines – two lines which intersect to form right angles

Pi – the ratio of the circumference of a circle to its diameter; typically we use 3.14 or 22/7 as an approximation of pi

Plane of symmetry – a three-dimensional figure has a plane of symmetry when every point of the figure on one side of the plane has a mirror image on the other side of the plane

Point symmetry – any figure that has 180 degree rotational symmetry is said to have point symmetry about the turn center

Polygon – a simple, closed polygonal curve

Polyhedron – a simple closed surface made up of polygonal regions

Prism – a polyhedron in which the two congruent bases lie in parallel planes and the other faces are bounded by parallelograms

Pyramid – a polyhedron with one base and lateral faces which are triangles that come together at a point

Q

Quadrilateral – a polygon with 4 sides

R

Rectangle – a parallelogram with four right angles

Reflection – also called a flip; a motion which pairs each point of a figure with a corresponding point in a new figure in such a way that each line connecting the pairs of points is perpendicular to the line of reflection

Regular polygon – a polygon in which all sides are congruent and all interior angles are congruent

Regular polyhedron – a convex polyhedron whose faces are congruent regular polygonal regions such that the number of edges that meet at each vertex is the same for all of the vertices of the polyhedron

Regular tessellation – a tessellation which involves copies of a regular polygon

Rhombus – a parallelogram with all four sides congruent

Right angle – an angle which measures exactly 90 degrees

Right cylinder/cone – a cylinder/cone which does not “lean”

Right prism – a prism whose lateral faces are rectangles; a prism that is not “leaning”

Right triangle – a triangle with exactly one right angle

Rigid motion – see isometry

Rotation – a motion determined by holding one point – the center – fixed and rotating the figure about this point by a certain amount in a certain direction; also called a turn

Rotational symmetry – also called turn symmetry; when a figure can be rotated about some point less than 360 degrees and the figure falls on top of itself

S

Scale factor – the ration of the corresponding sides in similar figures

Scalene triangle – a triangle in which all three sides have different lengths

Semicircle – an arc which is half of a circle; an arc whose central angle is 180 degrees

Similar – two objects are similar if they have the same shaped but not necessarilty the same size

Simple closed surface – has exactly one interior, has no holes, and is hollow

Size transformation – a transformation which creates a figure similar to the original figure

Skew lines – two lines which cannot lie in the same plane (they do not intersect but they are not parallel either since they are in different planes)

Slide – see translation

Sphere – the set of all points in space that are a given distance from a given point (the center); it looks like a ball

Square – a rectangle with 4 congruent sides

Straight angle – an angle which measure exactly 180 degrees

Supplementary angles – a pair of angles whose measures sum to 180 degrees

T

Tessellation – covering a plane with copies of a figure in such a way that there are no gaps and no overlaps

Tetrahedron – a regular polyhedron with 4 triangle-shaped sides

Translation – also called a slide; a motion that involves moving every point of a figure a specified distance and a specified direction along a straight line

Transversal – a line that intersects a pair of lines

Trapezoid – a quadrilateral with exactly one pair of parallel sides

Triangle – a polygon with 3 sides

Turn – see rotation

Turn symmetry – see rotational symmetry

U

V

Vertex – the point of a cone

Vertical angles – a pair of angles which lie across from each other when two lines intersect

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