Terminology 
Definition 
Illustration 
Acute Angle 
Angle with a measure of less than 90 degrees 




Acute Triangle 
A triangle with all interior angles measuring less than 90 degrees 




Adjacent Angles 
Two angles with a common side and a common vertex (corner point) that do not overlap 




Angle 
Two rays that share an endpoint 




Arc 
Part of a circle's curve between any two of its points 




Collinear 
Three or more points on a straight line 




Complementary Angles 
Two angles with a sum of 90 degrees 




Degree Measure 
Unit for measuring angles based on dividing one circle into 360 equal parts 




Diagonal 
Straight lines joining two opposite corners of a straightsided shape 




Equilateral Triangle 
Triangle with three equal sides and three equal angles 




Figure 
Set of points in the plane 




Interior of an Angle 
The convex region defined by the angle 




Intersecting Lines 
Lines that contain at least one point in common 




Isosceles Triangle 
Triangle with at least two equal sides 




Length of an Arc 
Circular distance around the arc 




Line 
A straight path that extends without end in both directions 




Line of Symmetry 
A line that divides a figure into two congruent halves that are mirror images of each other 




Line Segment 
A part of a line with two endpoints 




Obtuse Angle 
An angle with a measure greater than 90 degrees but less than 180 degrees 




Obtuse Triangle 
A triangle with an interior obtuse angle 




Parallel 
Two lines in a plane that do not intersect 




Perpendicular 
Two intersecting lines that form right (90 degree) angles 




Point 
Precise location on a plane 




Protractor 
Instrument used in measuring or sketching angles 




Ray 
Part of a line that has one endpoint and goes on forever in one direction 




Right Angle 
Angle formed by perpendicular lines, measuring 90 degrees 




Right Triangle 
A triangle that contains one 90 degree angle 




Scalene Triangle 
A triangle with no sides or angles equal 




Straight Angle 
Angle that measures 180 degrees 




Supplementary Angles 
Two angles with a sum of 180 degrees 




Triangle 
Consists of three noncollinear points and three line segments between them, the three segments are called sides and the three points are called vertices 




Vertex 
A point at which two line segments, lines, or rays meet to form an angle 




Vertical Angle 
When two lines intersect, any two nonadjacent angles formed by those lines are called vertical angles or vertically opposite angles 
