Isosceles Triangles 

An isosceles triangle is a triangle that has two congruent sides.  

Terminology 

The two congruent sides are called legs. The remaining side is called the base. The angles opposite the two congruent sides are called base angles. The angle opposite of the base is called the vertex angle.

An Important Property of Isosceles Triangles

In an isosceles triangle, the angles opposite of the two congruent sides are congruent. That is, the base angles of an isosceles triangle are congruent.

Other Properties

1. The altitude drawn from the vertex angle to the base is also the median and the angle bisector.
2. The other two angles must be acute and are complementary (i.e., add up to 90°).

Notes

1. An equilateral/equiangular triangle can be classified as an isosceles triangle.