What type of triangle has all sides of different lengths?

A triangle with all sides of different lengths is classified as a scalene triangle. In a scalene triangle, not only are the sides different, but the angles are also unique, setting it apart from other triangle types.

An equilateral triangle, for instance, has all sides of equal length, which directly contradicts the definition of a scalene triangle. An isosceles triangle features at least two sides that are of equal length, and therefore cannot have all sides of varying lengths. As for a right triangle, that classification refers to the angle measurement (one angle being exactly 90 degrees) and can include many configurations of side lengths, including scalene triangles.

Thus, the definition of a scalene triangle directly aligns with the requirement that all sides be of different lengths, making it the correct choice for this question.