Basic Definitions in Geometry

1. Basic Definitions in Geometry

2. Acute Angle • An angle that measures between 0 and 90 degrees.

3. Right Angle • Angle that measures 90 degrees and formed by perpendicular lines.

4. Obtuse Angle • An angle that measures between 90 and 180 degrees.

5. Straight Angle • An angle that measures 180 degrees.

6. Reflex Angle • An angle that measures between 180 and 360 degrees.

7. Complementary Angle • Two angles that add up to 90 degrees.

8. Supplementary Angles • Two angles that add up to 180 degrees.

9. Equilateral Triangle • A triangle that has 3 equal sides.

10. Equiangular Triangle • A triangle that has 3 equal angles and each measures 60 degrees (sum is 180).

11. Scalene Triangle • Triangle with different sides and angles.

12. Isosceles Triangle • A triangle that has two equal sides and two equal base angles. 

13. Vertical Angles • Two angles in which the sides of one angle are opposite rays to the sides of the second angle.

14. Adjacent Angles • Two angles having a common vertex and a common side, but do not have any interior points in common.

15. Line Segment • 2 points called endpoints and all of the points in between them.

16. Ray • A part of a line that consists of a point on the line, called the endpoint, and all of the points on one side of the line.

17. Opposite Rays • Two rays of the same line with a common endpoint and no other points in common.

18. Collinear • A set of points all lying on the same straight line.

19. Midpoint • The point of a line segment that divides it into 2 congruent (equal) parts.

20. Bisector • A line, ray, or segment which divides a segment into 2 congruent parts.

21. Perpendicular Lines • Two lines that intersect to form right angles.

22. Altitude • The perpendicular segment from a vertex of a polygon to the opposite side.

23. Median • A segment that joins any vertex of a triangle to the midpoint of the opposite side.