What kind of triangle has exactly two equal sides?

A triangle that has exactly two equal sides is known as an isosceles triangle. The defining characteristic of isosceles triangles is that they have two sides of equal length, which also means that the angles opposite those sides will be equal. This property not only distinguishes isosceles triangles from other types but also allows for various geometric applications, such as calculating angles and finding area when additional information is provided.

Equilateral triangles, on the other hand, have all three sides of equal length, making them a specific case of isosceles triangles but not fitting the criteria of having exactly two sides equal. Scalene triangles are characterized by having all sides of different lengths, and thus do not meet the condition for two equal sides. Acute triangles refer to the angles within a triangle, specifically those where all angles are less than 90 degrees, but this classification does not involve side lengths directly.

Therefore, the definition of an isosceles triangle clearly aligns with the criteria outlined in the question.