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Unit 1: Foundations of Geometry
Vocabulary
1. Acute angle: An angle with measures between 0° and 90°.
2. Adjacent angles: Two angles that share a common vertex and side, but have no common interior
points.
3. Angle: Consists of two different rays with the same endpoint. The rays are the sides of the angle, and
the endpoint is the vertex of the angle.
4. Between: When three points lie on a line, you cans say that one point is between the other two.
5. Bisector of a segment: A point, ray, line, segment, or plane that intersects a segment at its midpoint.
6. Bisector of an angle: A ray that divides an angle into two angles that are congruent.
7. Collinear points: Points that lie on the same line.
8. Complementary angles: Two angles whose measures have the sum of 90°. The sum of the measures of
an angle and its complement is 90°.
9. Congruent angles: Angles that have the same measure.
10. Coplanar: Lie on the same plane.
11. Defined terms: Terms that can be described using known words.
12. Degree measure: Angle measurement on a protractor numbered 0 to 180.
13. Distance: The absolute value of the difference of the coordinates of two points.
14. Euclidean geometry: Study of geometry based on undefined terms (point, line, plane), definitions,
postulates (axioms), and assumptions of the mathematician Euclid.
15. Exterior of an angle: Outside the two rays that form an angle.
16. Interior of an angle: Inside the two rays that form an angle.
17. Intersection: The set of points that two or more geometric figures have in common.
18. Length: The straight distance from one point to another.
19. Line: A line has one dimension. It is usually represented by a straight line with two arrowheads to
indicate that the line extends without end in two directions.
20. Line segment: Part of a line that consists of two points, called endpoints, and all points on the line that
are between the endpoints.
21. Linear pair of angles: Two adjacent angles whose noncommon sides are opposite rays.
22. Non-collinear: Not on the same line.
23. Non-coplanar: Not in the same plane.
24. Non-Euclidean geometry: Forms of geometry that contain a postulate that negate the Euclidean
parallel postulate.
25. Obtuse angle: An angle with measure between 90° and 180°.
26. Parallel lines: Two lines that do not intersect and are coplanar.
27. Perpendicular lines: Two lines that intersect to form a right angle.
28. Plane: a plane has two dimensions. It is usually represented by a shape that looks like a floor or a wall.
You must imagine that the plane extends without end, even though the drawing of a plane appears to
have edges.
29. Point: A point has no dimension. It is usually represented by a dot.
30. Postulate: A rule that is accepted without proof.
31. Ray: Part of a line that consists of a point called an endpoint and all points on the line that extend in
one direction.
32. Right angle: An angle with measure equal to 90°.
33. Skew lines: Lines that do not intersect and are not coplanar.
34. Space: Set of all points, boundless and three-dimensional.
35. Straight angle: An angle with measure equal to 180°.
36. Supplementary angles: Two angles whose measures have the sum of 180°.
37. Taxicab geometry: A non-Euclidean geometry in which all lines are horizontal or vertical.
38. Theorem: A true statement that follows as a result of other true statements.
39. Undefined term: A word that does not have a formal definition, but there is agreement about what the
word means.
40. Vertex of an angle: The point where the two rays meet.
41. Vertical angles: Two angles whose sides form two pairs of opposite rays.