In the mathematical theory of dynamical systems, an isochron is a set of initial conditions for the system that all lead to the same long-term behaviour.[1][2]
^J. Guckenheimer, Isochrons and phaseless sets, J. Math. Biol., 1:259–273 (1975)
^S.M. Cox and A.J. Roberts, Initial conditions for models of dynamical systems, Physica D, 85:126–141 (1995)