Which is the correct definition of an isosceles triangle?

An isosceles triangle is indeed defined as a triangle that has two sides of equal length, which means it also has two angles that are equal in measure opposite those sides. This definition captures the essential characteristic of an isosceles triangle.

Understanding this concept is important in the study of triangle properties in geometry. It assists students in recognizing different types of triangles based on their side lengths and angle measures, which is fundamental to further geometric reasoning and proofs.

By focusing on the number of congruent sides, students can classify triangles correctly. This classification system not only facilitates their understanding of basic triangle properties but also serves as a foundation for exploring more complex geometric concepts later on.

In contrast, the other descriptions do not accurately represent an isosceles triangle. A triangle with no congruent sides describes a scalene triangle, while a triangle with three congruent sides refers to an equilateral triangle. A right triangle is specifically defined by one of its angles being 90 degrees, which does not relate to the congruency of its sides.