 Arf used his invariant, among others, in his endeavor to classify quadratic forms in characteristic 2.In the special case of the 2-element field F2 the Arf invariant can be described as the element of F2 that occurs most often among the values of the form.Two nonsingular quadratic forms over F2 are isomorphic if and only if they have the same dimension and the same Arf invariant.