Which triangle has at least two sides of equal length?

An isosceles triangle is defined by having at least two sides of equal length. This characteristic gives the isosceles triangle its name, as "iso" means equal in Greek, and "sceles" refers to legs or sides. In an isosceles triangle, the angles opposite the equal sides are also equal, contributing to the triangle's symmetry.

Equilateral triangles, while having three equal sides, are not specifically classified as having "at least two" equal sides, although they do meet that requirement as a subset of isosceles triangles. Right and obtuse triangles describe types of angles within the triangle rather than the lengths of the sides, so while they may or may not have equal sides, they do not inherently meet the criterion of having at least two sides of equal length. Therefore, the correct choice specifically highlights the defining feature of isosceles triangles.