Geometry covers a vast range of topics. In this lesson, we’ll learn vocabulary for a fundamental aspect of geometry: angles.

  • angle
  • intersect
  • ray
  • endpoint
  • vertex
  • degree
  • acute angle
  • right angle
  • obtuse angle
  • plane
  • translation
  • rotation
  • reflection
  • dilation
  • congruent shape
  • similar shape

Angles

An angle is the space between either:

two lines that intersect
angle 1
In this picture, AB and CD are lines that intersect (cross). The spaces between are angles. For example, the space between A and D that's marked in green is an angle.
two rays that share an endpoint
angle 2
EK and GK are rays. Rays are "half lines." This means that they continue infinitely in one direction but have an endpoint. K is the endpoint for both EK and GK. The space between E and G is an angle.

When two rays share an endpoint, the endpoint is called a vertex. K is a vertex.

An angle is measured in degrees (like temperature!). A straight line is 180°.  An angle  is between 0° and 180°.

There are special names for three different kinds of angles. A right angle is exactly 90°.  An acute angle is less than 90°.  An obtuse angle is more than 90°.

types of angles

Congruent and similar shapes

It's important to remember that angles don't sit in nothingness. 2D figures like angles, circles, or triangles exist on a plane. A plane is an infinitely tall and wide, flat, 2D surface. Shapes can move on the plane. There are several ways a shape can move, and each way has a special name.

shapes
  • A translation is when a shape moves in one direction, but it doesn't flip or turn.
  • A rotation is when a shape "turns" or rotates.
  • A reflection is when a shape "flips" to make a mirror image of itself.

Unlock full access by logging in. Registered users can explore the entire lesson and more.

keyboard_arrow_up