What Is an Isosceles Triangle?
What Is an Isosceles Triangle?
An isosceles triangle is a triangle with two equal sides and two equal angles across from them.
Note: The lines through the two sides of the triangle in the image above indicate they are of equal length. The two arcs indicate the two angles are equal.
Dictionary Definition
The MerriamWebster dictionary defines an isosceles triangle as "a triangle in which two sides have the same length."
Real Examples of Isosceles Triangles
Below are real examples of isosceles triangles.
In each case

2 sides are the same length.

2 angles are the same.
The Parts of an Isosceles Triangle
An isosceles triangle has 2 equal sides. These sides are called legs.
The third side is called the base.
An isosceles triangle has 2 equal angles. These are the angles between each leg and the base. They are called base angles.
The angle between the legs is called the vertex angle.
Properties of Isosceles Triangles
The Symmetry of Isosceles Triangles
An isosceles triangle has 1 line of symmetry. It is the line that goes through the vertex angle and the midpoint of the base.