An isosceles triangle is a triangle with two congruent sides. The angles opposite these two sides are the base angles and the angle between those sides is the vertex angle of the triangle.

This definition implies that any equilateral triangle is isosceles too, but there are isosceles triangles that are not equilateral.

In any isosceles triangle, the angles opposite to the congruent sides are also congruent. In fact, this condition could be used to give an alternate definition of isosceles, since a triangle is isosceles if and only if it has two congruent angles.

In an equilateral triangle, the height, the median and the bisector to the third side are the same line.