A polygon with two sides equal in length.

(of a triangle) having two sides of equal length

is the union of the Greek iso (same or equal) and skelos (legs) and refers to two sides of a object as being the same length, as in isosceles triangles and isosceles trapezoids. The root iso shows up in many scientific and mathematical words such as isometry (same measure), and isomorphic (same shape). Isobar is used both in chemistry (two atoms with equal atomic weight) and in meteorology (lines connecting points of equal barometric pressure).

a triangle that has at least two congruent sides.

having a pair of base angles which are congruent, usually in reference to a triangle but also sometimes with reference to a trapezoid (as respectively depicted in the illustration, shown at right); moreover, the two corresponding sides are equal in length