What defines an equilateral triangle?

An equilateral triangle is defined by having three sides that are all of equal length. This property ensures that not only do all sides measure the same, but it also results in all three interior angles being equal, each measuring 60 degrees. This is a fundamental characteristic of equilateral triangles and is a direct consequence of the congruence of the sides. When all sides are equal, it naturally leads to equal angles opposite those sides, thus completing the definition of equilateral triangles.

The other descriptions provided in the options do not accurately capture the essence of an equilateral triangle. While it is true that an equilateral triangle has angles equal to 60 degrees, this is a consequence of the property stated, rather than the defining characteristic itself. Similarly, a triangle with at least one right angle or a triangle with two equal sides refer to different types of triangles and do not meet the criteria that define an equilateral triangle. Therefore, focusing on the property of equal side lengths correctly identifies the triangle as equilateral.