What defines a scalene triangle?

A scalene triangle is defined as one that has no congruent sides, meaning all three sides are of different lengths. This distinct characteristic sets it apart from other types of triangles. In an isosceles triangle, for instance, at least two sides are equal, while an equilateral triangle has all three sides the same length.

Additionally, the definitions of the other options clarify why they do not apply to scalene triangles. A triangle with all angles equal describes an equilateral triangle, and a triangle with two equal sides is specifically an isosceles triangle. Finally, a triangle with one angle measuring 90 degrees refers to a right triangle, which can be scalene or isosceles, but does not inherently qualify as scalene based on that angle alone. Thus, the defining feature of the scalene triangle is that it has no sides that are congruent.