What defines an isosceles triangle?

An isosceles triangle is defined by having two sides that are equal in length. This characteristic distinguishes isosceles triangles from other types of triangles. In addition to the two equal sides, the angles opposite these sides are also equal, which is a fundamental property of isosceles triangles. This is why the defining feature of an isosceles triangle is that it has two equal sides.

The other options describe different types of triangles or relationships between their sides and angles. For example, the option stating that all sides are equal refers to an equilateral triangle, where all three sides and all three angles are the same. The mention of one angle being 90 degrees describes a right triangle, which has a specific definition based on its angle rather than the equality of its sides. Lastly, stating that no sides are equal describes a scalene triangle, where all sides are of different lengths. Each of these options highlights a different type of triangle, making it clear why the correct definition of an isosceles triangle is the one that identifies it by having two equal sides.