In geometry, an isosceles triangle is a triangle that has two sides of equal length. Sometimes it is specified as having exactly two sides of equal length, and sometimes as having at least two sides of equal length, the latter version thus including the equilateral triangle as a special case. Examples of isosceles triangles include the isosceles right triangle, the golden triangle, and the faces of bipyramids and certain Catalan solids.

त्रिकोणाच्या तीन बाजूंपैकी दोन बाजूंची लांबी सारखीच असेल तर त्या‍ त्रिकोणास समद्विभुज त्रिकोण म्हणतात. समान बाजूंसमोरील कोन समान असतात, म्हणजेच या त्रिकोणातले दोन कोन सारख्या मापाचे असतात, आणि तिसरा वेगळ्या मापाचा.