Terminology
 New or Recently Introduced Terms 

  Acute angle (angle with a measure of less than 90°)

 Acute triangle (triangle with all interior angles measuring less than 90°)

 Adjacent angle (Two angles ∠??? and ∠???, with a common side ?? ⃑⃑⃑⃑⃑ , are adjacent angles if ? is in the interior of ∠???.)  Angle (union of two different rays sharing a common vertex, e.g., ∠???) 

 Arc (connected portion of a circle)       
 Collinear (Three or more points are collinear if there is a line containing all of the points; otherwise, the points are non-collinear.) 

 Complementary angles (two angles with a sum of 90°)

 Degree, degree measure of an angle (Subdivide the length around a circle into 360 arcs of equal length.

      A central angle for any of these arcs is called a one-degree angle and is said to have an angle measure of 1°. )

     Diagonal (straight lines joining two opposite corners of a straight-sided shape) 

    Equilateral triangle (triangle with three equal sides) 

     Figure (set of points in the plane) 

     Interior of an angle (the convex1 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 (straight path with no thickness that extends in both directions without end, e.g., ?? ⃡⃑⃑⃑ ⃑ )

 Line of symmetry (line through a figure such that when the figure is folded along the line, two halves are created that match up exactly) 

      Line segment (two points, ? and ?, together with the set of points on ?? ⃡⃑⃑⃑ ⃑  between ? and ?, e.g., ?? ̅̅̅̅) 

     Obtuse angle (angle with a measure greater than 90°, but less than 180°) 

     Obtuse triangle (triangle with an interior obtuse angle)

 Parallel (two lines in a plane that do not intersect, e.g., ?? ⃡⃑⃑⃑ ⃑  ∥ ?? ⃡⃑⃑⃑ ⃑ ) 

     Perpendicular (Two lines are perpendicular if they intersect, and any of the angles formed between the lines is a 90° angle, e.g., ?? ⃡⃑ ⃑ ⃑ ⃑  ⊥ ?? ⃡⃑⃑⃑⃑ .)

 Point (precise location in the plane) 

     Protractor (instrument used in measuring or sketching angles) 

     Ray (The ?? ⃑⃑⃑⃑⃑  is the point ? and the set of all points on ?? ⃡⃑⃑⃑⃑  that are on the same side of ? as the  point ?.) 

     Right angle (angle formed by perpendicular lines, measuring 90°)

 Right triangle (triangle that contains one 90° angle)

  Scalene triangle (triangle with no sides or angles equal) 

     Straight angle (angle that measures 180°)  

     Supplementary angles (two angles with a sum of 180°)

 Triangle (A triangle consists of three non-collinear points and the three line segments between them.  The three segments are called the sides of the triangle, and the three points are called the vertices.)

 Vertex (a point, often used to refer to the point where two lines meet, such as in an angle or the corner of a triangle)

 Vertical angles (When two lines intersect, any two non-adjacent angles formed by those lines are called vertical angles or vertically opposite angles.)