Definition of isosceles in English:



  • (of a triangle) having two sides of equal length.

    • ‘First construct an isosceles triangle whose base angles are double the vertex angle.’
    • ‘The base angles of an isosceles triangle are equal.’
    • ‘This is the ‘alternate’ solution to constructing the isosceles triangle with the base of the square as one side, using only a straightedge.’
    • ‘Mathematicians sometimes say that A is an arbitrary isosceles triangle, but all they mean is that A could be any such triangle.’
    • ‘The base of the wedge is an isosceles right triangle in a vertical plane.’


Mid 16th century: via late Latin from Greek isoskelēs, from isos equal + skelos leg.