What is the name for a triangle with two equal sides?

A triangle with two equal sides is called an isosceles triangle.

The two equal sides are called the legs, while the remaining side is called the base. The angles opposite the equal sides are also equal, a relationship known as the isosceles triangle theorem. This gives the shape a line of reflection symmetry running from the opposite vertex to the midpoint of the base.

The name comes from Greek roots meaning “equal” and “leg.” Isosceles triangles appear in architectural pediments, roof designs, geometric diagrams, and familiar shapes such as the golden triangle. An isosceles right triangle is a special example with two equal legs and a 90-degree angle.

One terminology detail can create confusion: some modern definitions say an isosceles triangle has at least two equal sides, which technically includes an equilateral triangle. Other textbooks use “isosceles” to mean exactly two equal sides. Either way, the standard answer to this question is Isosceles Triangle, not equilateral, right, or scalene triangle.