Which of the following describes an isosceles triangle?

An isosceles triangle is characterized by having at least two sides of equal length, which leads to the angles opposite those sides also being equal. This property is a defining trait of isosceles triangles and is essential for identifying them. The equal sides are commonly referred to as the legs of the triangle, while the third side is known as the base.

This definition is supported by the foundational principles of triangle geometry, which state that the angles opposite to equal sides must also be equal, establishing a direct correlation between side lengths and angle measures in isosceles triangles. Understanding this relationship is crucial when studying the properties of triangles, as it helps recognize various triangle types based on their side lengths and angles.

In contrast, the other definitions presented do not accurately describe isosceles triangles. For instance, having three sides of equal length refers to an equilateral triangle, while having no equal sides defines a scalene triangle. Additionally, stating that all angles are equal to 60 degrees is specific to equilateral triangles, not isosceles ones. Hence, the correct answer effectively encapsulates the essential properties that define an isosceles triangle.