What is the defining feature of an isosceles triangle?

An isosceles triangle is characterized by having two sides that are of equal length. This defining feature leads to several important properties: the angles opposite those two equal sides are also equal, which is a fundamental aspect of isosceles triangles. This symmetry and equality of the angles and sides contributes to various geometric applications and theorems involving isosceles triangles.

In contrast, the other choices describe different types of triangles. For instance, a triangle with all sides equal is classified as an equilateral triangle. Options referring to all sides being unequal or including a right angle pertain to scalene and right triangles respectively, neither of which align with the characteristics of an isosceles triangle. Thus, the correct answer emphasizes the specific property of having two sides that are equal in length, making it distinct from other triangle types.