Basic Definitions in Geometry

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

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

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

Straight Angle An angle that measures 180 degrees.

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

Complementary Angle Two angles that add up to 90 degrees.

Supplementary Angles Two angles that add up to 180 degrees.

Equilateral Triangle A triangle that has 3 equal sides.

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

Scalene Triangle Triangle with different sides and angles.

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

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

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

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

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.

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

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

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

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

Perpendicular Lines Two lines that intersect to form right angles.

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

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