isosceles

/aɪˌsɑsəˈliz/

In geometry, the word isosceles describes a triangle with two sides that are the exact same length. The angles across from each equal side are also the same in an isosceles triangle.

Mathematicians use the term isosceles in two slightly different ways. The Greek mathematician Euclid defined an isosceles triangle as having exactly (and only) two equal sides. Modern geometry experts tend to say that a triangle is isosceles if at least two sides are equal — making an equilateral triangle (with three equal sides) also an isosceles triangle. The Greek root, isoskeles, means "with equal legs."

Definitions of isosceles

1. adjective

   (of a triangle) having two sides of equal length
   synonyms:

   symmetric, symmetrical

   having similarity in size, shape, and relative position of corresponding parts