that has three equal sides. It is a special case of an isosceles triangle in the modern definition, stating that an isosceles triangle is defined at least as having two equal sides. Based on the modern definition, this leads to an equilateral triangle in which one of the three sides may be considered its base.

The follow-up definition above may result in more precise properties. For example, since the perimeter of an isosceles triangle is the sum of its two