** 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 ∠𝐴𝑂𝐶 and ∠𝐶𝑂𝐵, with a common side 𝑂𝐶 ⃑⃑⃑⃑⃑ , are adjacent angles if 𝐶 is in the interior of ∠𝐴𝑂𝐵.) Angle (union of two different rays sharing a common vertex, e.g., ∠𝐴𝐵𝐶)

** 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., 𝐴𝐵 ⃡⃑⃑⃑ ⃑ )

** 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, 𝐴 and 𝐵, together with the set of points on 𝐴𝐵 ⃡⃑⃑⃑ ⃑ between 𝐴 and 𝐵, e.g., 𝐴𝐵 ̅̅̅̅)

** 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., 𝐴𝐵 ⃡⃑⃑⃑ ⃑ ∥ 𝐶𝐷 ⃡⃑⃑⃑ ⃑ )

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

** Point** (precise location in the plane)

** Protractor** (instrument used in measuring or sketching angles)

** Ray** (The 𝑂𝐴 ⃑⃑⃑⃑⃑ is the point 𝑂 and the set of all points on 𝑂𝐴 ⃡⃑⃑⃑⃑ that are on the same side of 𝑂 as the point 𝐴.)

** 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.)