In [[quantum field theory]], a '''nonlinear ''σ'' model''' describes a [[scalar field]] {{mvar|Σ}} which takes on values in a nonlinear manifold called the [['''target manifold]]''' ''T''. The non-linear ''σ''-model was introduced by {{harvtxt|Gell-Mann|Lévy|1960|loc=section 6}}, who named it after a field corresponding to a spinless meson called ''σ'' in their model.<ref>{{Citation | last2=Lévy | first1=M. | last1=Gell-Mann | first2=M. | title=The axial vector current in beta decay | publisher=Italian Physical Society | doi=10.1007/BF02859738 | year=1960 | journal=Il Nuovo Cimento | issn=1827-6121 | volume=16 | pages=705–726}}</ref>
