A/An __________ triangle has two sides of equal length and a third side that is longer or shorter.

An isosceles triangle is defined by having at least two sides of equal length. The third side can be either longer or shorter than the equal sides, which is a defining characteristic of isosceles triangles. This unique feature enables isosceles triangles to have certain symmetrical properties, particularly in relation to their angles—specifically, the angles opposite the equal sides are also equal. This contrasts with other types of triangles, like equilateral triangles where all sides and angles are equal, or scalene triangles where all sides and angles are different, and right triangles which specifically include one 90-degree angle but do not necessarily have equal sides.