Equilateral describes a shape where all sides have exactly the same length.

An equilateral triangle is the most common example: all three sides measure exactly the same, and all three angles are exactly 60 degrees. If you draw an equilateral triangle with sides that are each 5 inches long, you could rotate it in any direction and it would look identical. This special property makes equilateral triangles important in art, architecture, and mathematics.

While the term applies mainly to triangles, you might also hear about equilateral polygons, meaning shapes with equal sides. However, mathematicians usually prefer more specific terms for other shapes: we call a four-sided equilateral shape a rhombus if the angles aren't right angles, or a square if they are.

Don't confuse equilateral with equiangular, which means all angles are equal. A rectangle has equal angles but not equal sides. An equilateral triangle happens to be both equilateral and equiangular, but many shapes are one or the other, not both.