Definitions

Acute Angle – angle with measure between 0 degrees and 90 degrees

Acute Triangle – a triangle with all angles less than 90 degrees

Alternate Exterior Angles – on outside, on opposite sides of transversal, not consecutive

Alternate Interior Angles – are inside, opposite sides of transversal, not consecutive

Altitude – a perpendicular segment to the base that shows height

Angle Bisector – a ray, starting at vertex, that divides the angle into 2 equal parts

Bi-Conditional Statement – if and only if statement when conditional and its converse are both true

Centroid – the point in a triangle where 3 medians meet; is the center of gravity

Circumcenter – the point of concurrency of 3 perpendicular bisectors

Circumscribed Circle – when a circle passes through each vertex of a polygon

Complementary – exactly 2 angles that add up to 90 degrees; don’t have to share vertex

Conditional Statement – if then statements

Congruent Polygons – polygons same size and same shape

Consecutive Interior Angles – inside, don’t share vertex, same side of transversal; are supplementary

Construct – you do not rely on measurements (from a ruler) but you use a compass and a

straightedge

Converse – a reverse statement

Corresponding Angles – angles that correspond

Counter Example – an example that proves something false

Diagonal – a segment connecting any two non-consecutive vertices

Draw – drawing the figure carefully and accurately using proper measurements

Equiangular – all angles are equal

Equilateral – all sides are equal

Incenter – the point of concurrency of 3 angle bisectors

Inscribed Circle – when a circle touches each side of a polygon

Isosceles Triangle – a triangle with at least two equal sides

Kite – a quadrilateral with exactly two pairs of congruent sides

Linear Pair – two angles that add up to 180 degrees; must share vertex; always supplementary

Locus – set of points or the “path”

Median – a segment that starts at a midpoint of one of the sides, extending to the opposite vertex

Midpoint of Segment – the center point of line segment that divides it into 2 equal parts

Obtuse Angle – an angle greater than 90 degrees, not exceeding 180 degrees

Obtuse Triangle – a triangle with one obtuse angle

Orthocenter – the point of concurrency of 3 altitudes

Parallel Lines – two or more lines on the same plane that never intersect

Parallelogram – a quadrilateral with exactly two sets of parallel sides

Perpendicular Bisector – a segment bisector that forms a 90-degree angle (is perpendicular to) through the midpoint

Perpendicular Lines – two intersecting lines that form all right angles, don’t have to be on same plane

Polygon – close geometric figure in a plane, formed by connecting line segments endpoint to endpoint with each segment intersecting exactly two others

Postulate – a statement accepted without proof

Rectangle – a quadrilateral that is equiangular

Regular – both equilateral and equiangular

Rhombus – a quadrilateral with all side congruent

Right Angle – angle with measure exactly 90 degrees

Right Triangle – a triangle with exactly one 90-degree angle

Scalene Triangle – a triangle with no equal sides

Segment Bisector – a line passing through the midpoint

Similar – two polygons are similar if and only if the corresponding angles are congruent

and the corresponding sides are proportional

Sketch – a freehand sketch of a shape

Square – a regular quadrilateral

Supplementary – exactly 2 angles that add up to 180 degrees; don’t have to share vertex

Theorem – a statement that can be proven

Trapezoid – a quadrilateral with exactly one set of parallel sides

Vertical Angles – two angles opposite to each other that share one vertex

Shapes

Number of Sides	Name of Shape
3	Triangle
4	Quadrilateral
5	Pentagon
6	Hexagon
7	Heptagon
8	Octagon
9	Nonagon
10	Decagon
11	Undecagon
12	Dodagon